Add GUI test for toggle filter and opacity

src/test/rustdoc-gui/toggle-docs.goml

@@ -40,3 +40,32 @@ assert-attribute-false: (
 click: "#toggle-all-docs"
 wait-for-text: ("#toggle-all-docs", "[−]")
 assert-attribute: ("details.rustdoc-toggle", {"open": ""}, ALL)
+
+// Checking the toggles style.
+show-text: true
+define-function: (
+    "check-color",
+    (theme, filter),
+    [
+        // Setting the theme.
+	("local-storage", {"rustdoc-theme": |theme|, "rustdoc-use-system-theme": "false"}),
+        // We reload the page so the local storage settings are being used.
+        ("reload"),
+
+        ("assert-css", ("details.rustdoc-toggle > summary::before", {
+            "opacity": "0.5",
+            "filter": |filter|,
+        }, ALL)),
+        ("move-cursor-to", "details.rustdoc-toggle summary"),
+        ("assert-css", ("details.rustdoc-toggle > summary:hover::before", {
+            "opacity": "1",
+            "filter": |filter|,
+        })),
+        // moving the cursor somewhere else to not mess with next function calls.
+        ("move-cursor-to", ".search-input"),
+    ]
+)
+
+call-function: ("check-color", {"theme": "ayu", "filter": "invert(1)"})
+call-function: ("check-color", {"theme": "dark", "filter": "invert(1)"})
+call-function: ("check-color", {"theme": "light", "filter": "none"})