fix(ui): use theme input color for search focus bg

[jamime]

Issue: #13388

What I did

I changed the Search component that is used in the sidebar to use the same color as form/Input, instead of the toolbar background color.

Theme	Before	After
light
dark

How to test

1. Create a custom theme with a barBg prop.

import {create} from '@storybook/theming/create';

const light = create({
  base: 'light',
  barBg: 'hotpink',
});

const dark = create({
  base: 'dark',
  barBg: 'hotpink',
});

2. Focus the Search component in the sidebar.

• Is this testable with Jest or Chromatic screenshots? No.
• Does this need a new example in the kitchen sink apps? No.
• Does this need an update to the documentation? No.

[jamime]

Closing, fixed in #13895.

[shilman]

Argh, sorry I missed this @jamime 🙈