Extend theme change GUI test

src/test/rustdoc-gui/theme-change.goml

@@ -9,3 +9,19 @@ click: "#theme-choices > button:last-child"
 wait-for: 500
 // should be the light theme so let's check the color
 assert-css: ("body", { "background-color": "rgb(255, 255, 255)" })
+
+goto: file://|DOC_PATH|/settings.html
+click: "#theme-light"
+wait-for: 500
+assert-css: ("body", { "background-color": "rgb(255, 255, 255)" })
+assert-local-storage: { "rustdoc-theme": "light" }
+
+click: "#theme-dark"
+wait-for: 500
+assert-css: ("body", { "background-color": "rgb(53, 53, 53)" })
+assert-local-storage: { "rustdoc-theme": "dark" }
+
+click: "#theme-ayu"
+wait-for: 500
+assert-css: ("body", { "background-color": "rgb(15, 20, 25)" })
+assert-local-storage: { "rustdoc-theme": "ayu" }