Rollup merge of rust-lang#113482 - GuillaumeGomez:migrate-gui-test-color-20, r=notriddle

Migrate GUI colors test to original CSS color format

Follow-up of rust-lang#111459.

r? ``@notriddle``

Author: matthiaskrgr

tests/rustdoc-gui/sidebar-source-code.goml

@@ -25,24 +25,24 @@ call-function: (
     "check-colors",
     {
         "theme": "ayu",
-        "color": "rgb(197, 197, 197)",
-        "background_color": "rgb(20, 25, 31)",
+        "color": "#c5c5c5",
+        "background_color": "#14191f",
     }
 )
 call-function: (
     "check-colors",
     {
         "theme": "dark",
-        "color": "rgb(221, 221, 221)",
-        "background_color": "rgb(80, 80, 80)",
+        "color": "#ddd",
+        "background_color": "#505050",
     }
 )
 call-function: (
     "check-colors",
     {
         "theme": "light",
-        "color": "rgb(0, 0, 0)",
-        "background_color": "rgb(245, 245, 245)",
+        "color": "black",
+        "background_color": "#F5F5F5",
     }
 )