Rollup merge of rust-lang#116106 - GuillaumeGomez:migrate-gui-test-color-42, r=notriddle

Migrate GUI colors test to original CSS color format

Follow-up of rust-lang#111459.

And with this one I'm finally done with this migration.

r? ``@notriddle``

Author: matthiaskrgr

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"})