UI: Fix custom theme hover-color inconsistency

[yoshi2no]

Issue: #22199

What I did

• Currently, a tint of the default color.secondary in base.ts (code/lib/theming/src/base.ts) is used as background color in the bar, so changed to use theme.color.secondary as background color when BranchNode is hovered or focused.
• The same applies to LeafNode

※ The reason why 0.93 is passed for the first argument of the transparentize function is because the value of background.hoverable in base.ts (code/lib/theming/src/base.ts) did so.

BranchNode(before)	BranchNode(after)	LeafNode(before)	LeafNode(after)

How to test

• generate a new theme using the create() function from storybook/theming as folows.

// code/ui/.storybook/manager.ts
import { addons } from '@storybook/manager-api';
import startCase from 'lodash/startCase.js';
import { create } from '@storybook/theming/create';

addons.setConfig({
  sidebar: {
    renderLabel: ({ name, type }) => (type === 'story' ? name : startCase(name)),
  },
  theme: create({
    base: 'light',
    colorSecondary: '#FF8688',
  }),
});

• cd code
• yarn storybook:ui
• Check that the secondary-based color is used as background color when the BranchNode/LeafNode is hovered or focused.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[cdedreuille]

Thanks a lot for this change. I think this change makes a lot of sense.