Implement dark mode using color-scheme CSS property

src/client/accounts/accounts-signature-web-view.vala

@@ -12,18 +12,23 @@ public class Accounts.SignatureWebView : Components.WebView {
 
 
     private static WebKit.UserScript? app_script = null;
+    private static WebKit.UserStyleSheet? app_stylesheet = null;
 
     public static new void load_resources()
         throws GLib.Error {
         SignatureWebView.app_script = Components.WebView.load_app_script(
             "signature-web-view.js"
         );
+        SignatureWebView.app_stylesheet = Components.WebView.load_app_stylesheet(
+            "signature-web-view.css"
+        );
     }
 
 
     public SignatureWebView(Application.Configuration config) {
         base(config);
         this.user_content_manager.add_script(SignatureWebView.app_script);
+        this.user_content_manager.add_style_sheet(SignatureWebView.app_stylesheet);
     }
 
 }

ui/composer-web-view.css

@@ -3,12 +3,22 @@
  * Copyright 2017 Michael Gratton <mike@vee.net>
  */
 
+:root {
+  /* Adjust default controls and system colors for light and dark mode.
+  See https://www.w3.org/TR/css-color-adjust-1/#color-scheme-effect */
+  color-scheme: light dark;
+
+  /* Apply canvas background to the root element, so body can darken
+  it using alpha. */
+  background-color: canvas;
+}
+
 body {
+  background-color: rgb(50% 50% 50% / .05);
+
   margin: 0 !important;
   border: 0 !important;
   padding: 0 !important;
-  color: black;
-  background-color: #f9f9f9;
   font-size: medium !important;
 }
 
@@ -40,7 +50,7 @@ body > div#geary-quote {
 }
 
 body > div:focus-within {
-  background-color: white;
+  background-color: canvas;
 }
 
 body > div#geary-signature:focus-within,

ui/conversation-web-view.css

@@ -13,9 +13,10 @@
   transition: height 0.25s !important;
 }
 
-html {
-  color: black;
-  background-color: white;
+:root {
+  /* Adjust default controls and system colors for light and dark mode.
+  See https://www.w3.org/TR/css-color-adjust-1/#color-scheme-effect */
+  color-scheme: light dark;
 
   /* Width must always be defined by the viewport so content doesn't
   overflow inside the WebView, height must always be defined by the

ui/org.gnome.Geary.gresource.xml

@@ -49,6 +49,7 @@
     <file compressed="true" preprocess="xml-stripblanks">gtk/help-overlay.ui</file>
     <file compressed="true" preprocess="xml-stripblanks">password-dialog.glade</file>
     <file compressed="true" preprocess="xml-stripblanks">problem-details-dialog.ui</file>
+    <file compressed="true">signature-web-view.css</file>
     <file compressed="true">signature-web-view.js</file>
     <file compressed="true">geary.css</file>
     <file compressed="true">single-key-shortcuts.css</file>

ui/signature-web-view.css

@@ -0,0 +1,5 @@
+:root {
+  /* Adjust default controls and system colors for light and dark mode.
+  See https://www.w3.org/TR/css-color-adjust-1/#color-scheme-effect */
+  color-scheme: light dark;
+}