gui, lib/api: Adds support for prefers-color-scheme (fixes #6115)

* gui, lib/api: Adds support for prefers-color-scheme on default theme (fixes #6115)

- Renames current default theme into a new "light" theme
- Modifies assets serving to allow getting assets from different themes

* lib/api: Serve assets from arbitrary theme when path starts with "theme-assets"

* lib/api: Moves constant out of function

* Loads light theme in browsers without support for prefers-color-scheme

* gui: Disables dark theme when printing

* Prevents repeated injection and adds support for older browsers

The CSS is always loaded if there is no support for `matchMedia`.

gui/default/assets/css/theme.css

@@ -7,29 +7,5 @@
 
 */
 
-.panel-progress {
+@import "/theme-assets/dark/assets/css/theme.css" screen and (prefers-color-scheme: dark);
-    background: #3498db;
+@import "/theme-assets/light/assets/css/theme.css" (prefers-color-scheme: light), (prefers-color-scheme: no-preference);
-}
-
-.identicon rect {
-    fill: #333;
-}
-
-.panel-warning .identicon rect {
-    fill: #fff;
-}
-
-.li-column {
-    background-color: rgb(236, 240, 241);
-    border-radius: 3px;
-}
-
-.panel-heading:hover, .panel-heading:focus {
-    text-decoration: none;
-}
-
-.fancytree-ext-filter-hide tr.fancytree-submatch span.fancytree-title,
-.fancytree-ext-filter-hide span.fancytree-node.fancytree-submatch span.fancytree-title {
-    color: black !important;
-    font-weight: lighter !important;
-}

gui/default/syncthing/core/syncthingController.js

@@ -389,6 +389,7 @@ angular.module('syncthing.core')
             });
 
             refreshNoAuthWarning();
+            setDefaultTheme();
 
             if (!hasConfig) {
                 $scope.$emit('ConfigLoaded');
@@ -651,6 +652,23 @@ angular.module('syncthing.core')
             $scope.remoteNeedDevice = undefined;
         }
 
+
+        function setDefaultTheme() {
+            if (!document.getElementById("fallback-theme-css")){
+
+                // check if no support for prefers-color-scheme
+                var colorSchemeNotSupported = typeof window.matchMedia === "undefined" || window.matchMedia('(prefers-color-scheme: dark)').media === 'not all';
+
+                if ($scope.config.gui.theme === "default" && colorSchemeNotSupported) {
+                    document.documentElement.style.display = 'none';
+                    document.head.insertAdjacentHTML(
+                      'beforeend',
+                      '<link id="fallback-theme-css" rel="stylesheet" href="/theme-assets/light/assets/css/theme.css" onload="document.documentElement.style.display = \'\'">'
+                    );
+                }
+            }
+        }
+
         function saveIgnores(ignores, cb) {
             $http.post(urlbase + '/db/ignores?folder=' + encodeURIComponent($scope.currentFolder.id), {
                 ignore: ignores

gui/light/assets/css/theme.css

@@ -0,0 +1,35 @@
+/*
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+*/
+
+.panel-progress {
+    background: #3498db;
+}
+
+.identicon rect {
+    fill: #333;
+}
+
+.panel-warning .identicon rect {
+    fill: #fff;
+}
+
+.li-column {
+    background-color: rgb(236, 240, 241);
+    border-radius: 3px;
+}
+
+.panel-heading:hover, .panel-heading:focus {
+    text-decoration: none;
+}
+
+.fancytree-ext-filter-hide tr.fancytree-submatch span.fancytree-title,
+.fancytree-ext-filter-hide span.fancytree-node.fancytree-submatch span.fancytree-title {
+    color: black !important;
+    font-weight: lighter !important;
+}

lib/api/api_statics.go

@@ -23,6 +23,8 @@ import (
 	"github.com/syncthing/syncthing/lib/sync"
 )
 
+const themePrefix = "theme-assets/"
+
 type staticsServer struct {
 	assetDir        string
 	assets          map[string][]byte
@@ -91,6 +93,20 @@ func (s *staticsServer) serveAsset(w http.ResponseWriter, r *http.Request) {
 	modificationTime := s.lastThemeChange
 	s.mut.RUnlock()
 
+	// If path starts with special prefix, get theme and file from path
+	if strings.HasPrefix(file, themePrefix) {
+		path := file[len(themePrefix):]
+		i := strings.IndexRune(path, '/')
+
+		if i == -1 {
+			http.NotFound(w, r)
+			return
+		}
+
+		theme = path[:i]
+		file = path[i+1:]
+	}
+
 	// Check for an override for the current theme.
 	if s.assetDir != "" {
 		p := filepath.Join(s.assetDir, theme, filepath.FromSlash(file))