Firefox: Option Page Bug Fix

[ZimCodes]

Fixes enabling/disabling of the textbox selection/scrollbar options.

Description

Toggling the textbox selection and/or scrollbar options in the option page would break the current theme. This pull request also makes the search bar rounder.

Motivation and Context

This fixes a major bug which interferes with the behavior of the extension.

Screenshots (if appropriate):

https://user-images.githubusercontent.com/23222943/140520616-e39becef-552f-4876-abb9-bd4572e1d87b.mp4

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Non-Functional Change (non-user facing changes)

Checklist:

• [x] I updated the version (base package.json and re-built the master extension).
• [x] I updated the changelog with the new functionality.