Style: custom theme UI fixes (#3284)

* style: fix `background-color` inconsistency in issue layout when custom theme is applied.

* fix: theme dropdown overlapping with input color picker icons (z-index issue).

web/components/core/theme/theme-switch.tsx

@@ -46,7 +46,7 @@ export const ThemeSwitch: FC<Props> = (props) => {
       }
       onChange={onChange}
       input
-      width="w-full"
+      width="w-full z-20"
     >
       {THEME_OPTIONS.map((themeOption) => (
         <CustomSelect.Option key={themeOption.value} value={themeOption}>

web/components/issues/issue-layouts/roots/project-layout-root.tsx

@@ -48,7 +48,7 @@ export const ProjectLayoutRoot: React.FC = observer(() => {
           {Object.keys(getIssues ?? {}).length == 0 ? (
             <ProjectEmptyState />
           ) : (
-            <div className="relative h-full w-full overflow-auto">
+            <div className="relative h-full w-full overflow-auto bg-custom-background-90">
               {activeLayout === "list" ? (
                 <ListLayout />
               ) : activeLayout === "kanban" ? (