chore: add explicit return type to getBackgroundColor (#18878)

Trivial typing improvement to trigger CI Front + CI UI visual regression
pipelines.

.github/workflows/ci-ui.yaml

@@ -79,6 +79,8 @@ jobs:
           fetch-depth: 10
       - name: Install dependencies
         uses: ./.github/actions/yarn-install
+      - name: Build dependencies
+        run: npx nx build twenty-shared
       - name: Download storybook build
         uses: actions/download-artifact@v4
         with:

packages/twenty-ui/src/testing/decorators/ComponentDecorator.tsx

@@ -4,7 +4,10 @@ import { themeCssVariables } from '@ui/theme-constants';
 
 import { ComponentStorybookLayout } from '../ComponentStorybookLayout';
 
-const getBackgroundColor = (inverted: boolean, accent: string) => {
+const getBackgroundColor = (
+  inverted: boolean,
+  accent: string,
+): string | undefined => {
   if (!inverted) return undefined;
 
   switch (accent) {