search ui: change sidebar bg to white in light mode

Created by: limitedmage

Fixes https://www.figma.com/file/8MkIcpDtjv6tqgxZFcEQDt?node-id=832:25101#230899118

Change sidebar color to white in light mode when simple UI toggle is on

Test plan

CSS-only change, verify manually

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.