Add GUI test to prevent sidebar background regression

Author: GuillaumeGomez

src/test/rustdoc-gui/sidebar-mobile.goml

@@ -40,3 +40,25 @@ assert-position: ("#method\.must_use", {"y": 45})
 click: ".sidebar-menu-toggle"
 scroll-to: ".block.keyword li:nth-child(1)"
 compare-elements-position-near: (".block.keyword li:nth-child(1)", ".mobile-topbar", {"y": 543})
+
+// Now checking the background color of the sidebar.
+local-storage: {"rustdoc-use-system-theme": "false", "rustdoc-theme": "dark"}
+reload:
+
+// Open the sidebar menu.
+click: ".sidebar-menu-toggle"
+assert-css: (".sidebar", {"background-color": "rgb(80, 80, 80)"})
+
+local-storage: {"rustdoc-use-system-theme": "false", "rustdoc-theme": "ayu"}
+reload:
+
+// Open the sidebar menu.
+click: ".sidebar-menu-toggle"
+assert-css: (".sidebar", {"background-color": "rgb(20, 25, 31)"})
+
+local-storage: {"rustdoc-use-system-theme": "false", "rustdoc-theme": "light"}
+reload:
+
+// Open the sidebar menu.
+click: ".sidebar-menu-toggle"
+assert-css: (".sidebar", {"background-color": "rgb(245, 245, 245)"})