diff --git a/.github/manual-test b/.github/manual-test
index 1bf5ab7..db2dca4 100755
--- a/.github/manual-test
+++ b/.github/manual-test
@@ -9,9 +9,11 @@ if ! command -v node > /dev/null; then echo "Missing node"; missingDeps=true; fi
 if [ "$missingDeps" = true ]; then exit 1; fi
 
 function launch_app() {
-  printf '\n*** Running appn'
+  printf '\n*** Running app\n'
   if [ "$(uname -s)" = "Darwin" ]; then
     open -a "$1/$2-darwin-x64/$2.app"
+  elif [ "$(uname -o)" = "Msys" ]; then
+    "$1/$2-win32-x64/$2.exe" --verbose
   else
     "$1/$2-linux-x64/$2"
   fi
diff --git a/app/package.json b/app/package.json
index fb4a08e..cbb18a2 100644
--- a/app/package.json
+++ b/app/package.json
@@ -20,6 +20,6 @@
     "source-map-support": "^0.5.19"
   },
   "devDependencies": {
-    "electron": "^12.0.7"
+    "electron": "^12.0.11"
   }
 }
diff --git a/app/src/helpers/windowHelpers.test.ts b/app/src/helpers/windowHelpers.test.ts
index ee12665..0ec7327 100644
--- a/app/src/helpers/windowHelpers.test.ts
+++ b/app/src/helpers/windowHelpers.test.ts
@@ -97,6 +97,8 @@ describe('createNewTab', () => {
 });
 
 describe('injectCSS', () => {
+  jest.setTimeout(10000);
+
   const mockGetCSSToInject: jest.SpyInstance = getCSSToInject as jest.Mock;
   const mockLogError: jest.SpyInstance = error as jest.Mock;
   const mockWebContentsInsertCSS: jest.SpyInstance = jest.spyOn(
@@ -105,12 +107,13 @@ describe('injectCSS', () => {
   );
 
   const css = 'body { color: white; }';
-  const responseHeaders = { 'x-header': 'value' };
+  let responseHeaders;
 
   beforeEach(() => {
     mockGetCSSToInject.mockReset().mockReturnValue('');
     mockLogError.mockReset();
     mockWebContentsInsertCSS.mockReset().mockResolvedValue(undefined);
+    responseHeaders = { 'x-header': 'value', 'content-type': ['test/other'] };
   });
 
   afterAll(() => {
@@ -181,22 +184,34 @@ describe('injectCSS', () => {
     );
   });
 
-  test.each<string | jest.DoneCallback>(['DELETE', 'OPTIONS'])(
-    'will not inject for method %s',
-    (method: string, done: jest.DoneCallback) => {
+  test.each<string | jest.DoneCallback>([
+    'application/json',
+    'font/woff2',
+    'image/png',
+  ])(
+    'will not inject for content-type %s',
+    (contentType: string, done: jest.DoneCallback) => {
       mockGetCSSToInject.mockReturnValue(css);
       const window = new BrowserWindow();
 
+      responseHeaders['content-type'] = [contentType];
+
       injectCSS(window);
 
       expect(mockGetCSSToInject).toHaveBeenCalled();
 
-      window.webContents.emit('did-navigate');
+      expect(window.webContents.emit('did-navigate')).toBe(true);
+      mockWebContentsInsertCSS.mockReset().mockResolvedValue(undefined);
       // @ts-ignore this function doesn't exist in the actual electron version, but will in our mock
       window.webContents.session.webRequest.send(
         'onHeadersReceived',
-        { responseHeaders, webContents: window.webContents, method },
+        {
+          responseHeaders,
+          webContents: window.webContents,
+          url: `test-${contentType}`,
+        },
         (result: HeadersReceivedResponse) => {
+          // insertCSS will still run once for the did-navigate
           expect(mockWebContentsInsertCSS).not.toHaveBeenCalled();
           expect(result.cancel).toBe(false);
           expect(result.responseHeaders).toBe(responseHeaders);
@@ -206,23 +221,30 @@ describe('injectCSS', () => {
     },
   );
 
-  test.each<string | jest.DoneCallback>(['GET', 'PATCH', 'POST', 'PUT'])(
-    'will inject for method %s',
-    (method: string, done: jest.DoneCallback) => {
+  test.each<string | jest.DoneCallback>(['text/html'])(
+    'will inject for content-type %s',
+    (contentType: string, done: jest.DoneCallback) => {
       mockGetCSSToInject.mockReturnValue(css);
       const window = new BrowserWindow();
 
+      responseHeaders['content-type'] = [contentType];
+
       injectCSS(window);
 
       expect(mockGetCSSToInject).toHaveBeenCalled();
 
       window.webContents.emit('did-navigate');
+      mockWebContentsInsertCSS.mockReset().mockResolvedValue(undefined);
       // @ts-ignore this function doesn't exist in the actual electron version, but will in our mock
       window.webContents.session.webRequest.send(
         'onHeadersReceived',
-        { responseHeaders, webContents: window.webContents, method },
+        {
+          responseHeaders,
+          webContents: window.webContents,
+          url: `test-${contentType}`,
+        },
         (result: HeadersReceivedResponse) => {
-          expect(mockWebContentsInsertCSS).toHaveBeenCalled();
+          expect(mockWebContentsInsertCSS).toHaveBeenCalledTimes(1);
           expect(result.cancel).toBe(false);
           expect(result.responseHeaders).toBe(responseHeaders);
           done();
@@ -247,11 +269,18 @@ describe('injectCSS', () => {
       expect(mockGetCSSToInject).toHaveBeenCalled();
 
       window.webContents.emit('did-navigate');
+      mockWebContentsInsertCSS.mockReset().mockResolvedValue(undefined);
       // @ts-ignore this function doesn't exist in the actual electron version, but will in our mock
       window.webContents.session.webRequest.send(
         'onHeadersReceived',
-        { responseHeaders, webContents: window.webContents, resourceType },
+        {
+          responseHeaders,
+          webContents: window.webContents,
+          resourceType,
+          url: `test-${resourceType}`,
+        },
         (result: HeadersReceivedResponse) => {
+          // insertCSS will still run once for the did-navigate
           expect(mockWebContentsInsertCSS).not.toHaveBeenCalled();
           expect(result.cancel).toBe(false);
           expect(result.responseHeaders).toBe(responseHeaders);
@@ -272,12 +301,18 @@ describe('injectCSS', () => {
       expect(mockGetCSSToInject).toHaveBeenCalled();
 
       window.webContents.emit('did-navigate');
+      mockWebContentsInsertCSS.mockReset().mockResolvedValue(undefined);
       // @ts-ignore this function doesn't exist in the actual electron version, but will in our mock
       window.webContents.session.webRequest.send(
         'onHeadersReceived',
-        { responseHeaders, webContents: window.webContents, resourceType },
+        {
+          responseHeaders,
+          webContents: window.webContents,
+          resourceType,
+          url: `test-${resourceType}`,
+        },
         (result: HeadersReceivedResponse) => {
-          expect(mockWebContentsInsertCSS).toHaveBeenCalled();
+          expect(mockWebContentsInsertCSS).toHaveBeenCalledTimes(1);
           expect(result.cancel).toBe(false);
           expect(result.responseHeaders).toBe(responseHeaders);
           done();
diff --git a/app/src/helpers/windowHelpers.ts b/app/src/helpers/windowHelpers.ts
index cc42e24..b0eeefd 100644
--- a/app/src/helpers/windowHelpers.ts
+++ b/app/src/helpers/windowHelpers.ts
@@ -204,6 +204,13 @@ export function injectCSS(browserWindow: BrowserWindow): void {
       'browserWindow.webContents.did-navigate',
       browserWindow.webContents.getURL(),
     );
+
+    browserWindow.webContents
+      .insertCSS(cssToInject)
+      .catch((err: unknown) =>
+        log.error('browserWindow.webContents.insertCSS', err),
+      );
+
     // We must inject css early enough; so onHeadersReceived is a good place.
     // Will run multiple times, see `did-finish-load` event on the window
     // that unsets this handler.
@@ -213,7 +220,18 @@ export function injectCSS(browserWindow: BrowserWindow): void {
         details: OnHeadersReceivedListenerDetails,
         callback: (headersReceivedResponse: HeadersReceivedResponse) => void,
       ) => {
-        injectCSSIntoResponse(details, cssToInject)
+        const contentType =
+          'content-type' in details.responseHeaders
+            ? details.responseHeaders['content-type'][0]
+            : undefined;
+
+        log.debug('onHeadersReceived', {
+          contentType,
+          resourceType: details.resourceType,
+          url: details.url,
+        });
+
+        injectCSSIntoResponse(details, contentType, cssToInject)
           .then((responseHeaders) => {
             callback({
               cancel: false,
@@ -234,28 +252,33 @@ export function injectCSS(browserWindow: BrowserWindow): void {
 
 async function injectCSSIntoResponse(
   details: OnHeadersReceivedListenerDetails,
+  contentType: string,
   cssToInject: string,
 ): Promise<Record<string, string[]>> {
-  // We go with a denylist rather than a whitelist (e.g. only GET/html)
+  // We go with a denylist rather than a whitelist (e.g. only text/html)
   // to avoid "whoops I didn't think this should have been CSS-injected" cases
-  const nonInjectableMethods = ['DELETE', 'OPTIONS'];
+  const nonInjectableContentTypes = [
+    /application\/.*/,
+    /font\/.*/,
+    /image\/.*/,
+  ];
   const nonInjectableResourceTypes = ['image', 'script', 'stylesheet', 'xhr'];
 
   if (
-    nonInjectableMethods.includes(details.method) ||
+    nonInjectableContentTypes.filter((x) => x.exec(contentType)?.length > 0)
+      ?.length > 0 ||
     nonInjectableResourceTypes.includes(details.resourceType) ||
     !details.webContents
   ) {
     log.debug(
-      `Skipping CSS injection for:\n${details.url}\nwith method ${details.method} and resourceType ${details.resourceType} and content-type ${details.responseHeaders['content-type']}`,
+      `Skipping CSS injection for:\n${details.url}\nwith resourceType ${details.resourceType} and content-type ${contentType}`,
     );
     return details.responseHeaders;
   }
 
-  log.debug('browserWindow.webContents.session.webRequest.onHeadersReceived', {
-    details,
-    contentType: details.responseHeaders['content-type'],
-  });
+  log.debug(
+    `Injecting CSS for:\n${details.url}\nwith resourceType ${details.resourceType} and content-type ${contentType}`,
+  );
   await details.webContents.insertCSS(cssToInject);
 
   return details.responseHeaders;