fix: add `color-scheme` to themes

ui/static/css/dark.css

@@ -116,3 +116,7 @@
 
     --counter-color: #bbb;
 }
+
+html {
+    color-scheme: dark;
+}

ui/static/css/light.css

@@ -116,3 +116,7 @@
 
     --counter-color: #666;
 }
+
+html {
+    color-scheme: light;
+}

ui/static/css/system.css

@@ -116,6 +116,10 @@
     --counter-color: #666;
 }
 
+html {
+    color-scheme: light;
+}
+
 @media (prefers-color-scheme: dark) {
     :root {
         --body-color: #efefef;
@@ -233,4 +237,8 @@
 
         --counter-color: #bbb;
     }
+
+    html {
+        color-scheme: dark;
+    }
 }