WebUI: Add missing icons

This adds missing icons to WebUI (in tabs, buttons, etc.).

PR #21162.

src/webui/www/private/css/palette.css

@@ -16,6 +16,8 @@
     --color-background-hover: hsl(26deg 80% 60%);
     --color-border-blue: hsl(210deg 42% 48%);
     --color-border-default: hsl(0deg 0% 85%);
+    --color-icon-hover: brightness(0) invert(100%) sepia(100%) saturate(0%)
+        hue-rotate(108deg) brightness(104%) contrast(104%);
 }
 
 :root {
@@ -46,4 +48,11 @@
         --color-background-hover: hsl(26deg 50% 55%);
         --color-border-default: hsl(0deg 0% 33%);
     }
+
+    #rssButtonBar img,
+    #startSearchButton img,
+    #manageSearchPlugins img {
+        filter: brightness(0) saturate(100%) invert(100%) sepia(0%) saturate(1%)
+            hue-rotate(156deg) brightness(106%) contrast(101%);
+    }
 }