Add missing fallbacks, remove unused vars and mixins

[alex-page]

WHY are these changes introduced?

Fixes #0000

WHAT is this pull request doing?

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
    </Page>
  );
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide
• For visual design changes, pinged one of @ HYPD, @ mirualves, @ sarahill, or @ ry5n to update the Polaris UI kit

[github-actions]

🟡 This pull request modifies 8 files and might impact 15 other files. This is an average splash zone for a change, remember to tophat areas that could be affected.

Details:

All files potentially affected (total: 15)

• /src/components/Choice/Choice.tsx
• /src/components/Checkbox/Checkbox.tsx
• /src/components/CheckableButton/CheckableButton.tsx
• /src/components/BulkActions/BulkActions.tsx
• /src/components/ResourceList/ResourceList.tsx
• /src/components/ChoiceList/ChoiceList.tsx
• /src/components/ResourceItem/ResourceItem.tsx
• /src/components/RadioButton/RadioButton.tsx
• /src/components/DropZone/components/FileUpload/FileUpload.tsx
• /src/components/DropZone/DropZone.tsx
• /src/components/Frame/Frame.tsx
• /src/components/TopBar/components/SearchDismissOverlay/SearchDismissOverlay.tsx
• /src/components/TopBar/components/Search/Search.tsx
• /src/components/TopBar/TopBar.tsx
• /src/components/VideoThumbnail/VideoThumbnail.tsx

📄 UNRELEASED.md (total: 0)

Files potentially affected (total: 0)

🎨 src/components/Choice/Choice.scss (total: 8)

Files potentially affected (total: 8)

• /src/components/Choice/Choice.tsx
• /src/components/Checkbox/Checkbox.tsx
• /src/components/CheckableButton/CheckableButton.tsx
• /src/components/BulkActions/BulkActions.tsx
• /src/components/ResourceList/ResourceList.tsx
• /src/components/ChoiceList/ChoiceList.tsx
• /src/components/ResourceItem/ResourceItem.tsx
• /src/components/RadioButton/RadioButton.tsx

🎨 src/components/DropZone/components/FileUpload/FileUpload.scss (total: 2)

Files potentially affected (total: 2)

• /src/components/DropZone/components/FileUpload/FileUpload.tsx
• /src/components/DropZone/DropZone.tsx

🎨 src/components/Frame/Frame.scss (total: 1)

Files potentially affected (total: 1)

• /src/components/Frame/Frame.tsx

🎨 src/components/TopBar/components/SearchDismissOverlay/SearchDismissOverlay.scss (total: 3)

Files potentially affected (total: 3)

• /src/components/TopBar/components/SearchDismissOverlay/SearchDismissOverlay.tsx
• /src/components/TopBar/components/Search/Search.tsx
• /src/components/TopBar/TopBar.tsx

🎨 src/components/VideoThumbnail/VideoThumbnail.scss (total: 1)

Files potentially affected (total: 1)

• /src/components/VideoThumbnail/VideoThumbnail.tsx

🎨 src/styles/shared/_skeleton.scss (total: 0)

Files potentially affected (total: 0)

🎨 src/styles/shared/_typography.scss (total: 0)

Files potentially affected (total: 0)