Migrate GUI colors test to original CSS color format

Author: GuillaumeGomez

tests/rustdoc-gui/warning-block.goml

@@ -4,7 +4,7 @@ show-text: true
 
 define-function: (
     "check-warning",
-    (theme, color, border_color, background_color),
+    (theme, color, border_color),
     block {
         set-local-storage: {"rustdoc-theme": |theme|, "rustdoc-use-system-theme": "false"}
         reload:
@@ -27,19 +27,19 @@ define-function: (
 
 call-function: ("check-warning", {
     "theme": "ayu",
-    "color": "rgb(197, 197, 197)",
-    "border_color": "rgb(255, 142, 0)",
-    "background_color": "rgba(0, 0, 0, 0)",
+    "color": "#c5c5c5",
+    "border_color": "#ff8e00",
+    "background_color": "transparent",
 })
 call-function: ("check-warning", {
     "theme": "dark",
-    "color": "rgb(221, 221, 221)",
-    "border_color": "rgb(255, 142, 0)",
-    "background_color": "rgba(0, 0, 0, 0)",
+    "color": "#ddd",
+    "border_color": "#ff8e00",
+    "background_color": "transparent",
 })
 call-function: ("check-warning", {
     "theme": "light",
-    "color": "rgb(0, 0, 0)",
-    "border_color": "rgb(255, 142, 0)",
-    "background_color": "rgba(0, 0, 0, 0)",
+    "color": "black",
+    "border_color": "#ff8e00",
+    "background_color": "transparent",
 })