Firefox: Current tab's Toolbar theme fix in Mixed Mode

[ZimCodes]

The current active tab's toolbar theme becomes completely different from the tab's overall theme in Mixed Mode. This pull request corrects this issue.

Description

Originally, the toolbar theme of the current tab uses the very last tab's theme instead of currently active tab's theme. These changes makes sure to only set the toolbar theme of the currently active tab that the user is currently utilizing. See issue #77 for more info

Motivation and Context

Fixes #77 .

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)
• [ ] 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.