Migrate GUI colors test to original CSS color format

Author: GuillaumeGomez

tests/rustdoc-gui/default-settings.goml

@@ -5,4 +5,4 @@
 go-to: "file://" + |DOC_PATH| + "/settings/index.html"
 // Wait a bit to be sure the default theme is applied.
 // If the theme isn't applied, the command will time out.
-wait-for-css: ("body", {"background-color": "rgb(15, 20, 25)"})
+wait-for-css: ("body", {"background-color": "#0f1419"})