Rollup merge of #114537 - GuillaumeGomez:migrate-gui-test-color-29, r=notriddle

Migrate GUI colors test to original CSS color format

Follow-up of #111459.

r? ``@notriddle``

Author: matthiaskrgr

tests/rustdoc-gui/search-error.goml

@@ -20,20 +20,20 @@ call-function: (
     "check-colors",
     {
         "theme": "ayu",
-        "error_background": "rgb(79, 76, 76)",
+        "error_background": "#4f4c4c",
     },
 )
 call-function: (
     "check-colors",
     {
         "theme": "dark",
-        "error_background": "rgb(72, 72, 72)",
+        "error_background": "#484848",
     },
 )
 call-function: (
     "check-colors",
     {
         "theme": "light",
-        "error_background": "rgb(208, 204, 204)",
+        "error_background": "#d0cccc",
     },
 )