Added support for Firefox for Android

Adjusted width of popup window to 40em

Added syntax highlighting & line numbers using Prism (https://prismjs.com/)
Added more style to popup heading
Renamed 'Hostnames' to 'Domains' in Whitelist and Blacklist functionality description
Added label "Edit Custom CSS:"

Updated layout of popup by removing redundant text.
Fixed bug where thumbnail injector image wasn't showing.
Updated console log to be less ambiguous when 'saving' custom CSS.
Removed border-colour of custom CSS text-area turning red when applying custom CSS to URL or Domain to avoid confusion of an error occurring.

Updated the Toolbar icon graphic.

Added functionality to allow users to choose whether their custom CSS is applied to URL, Domain or Globally.
Removed options page (All options are now easily reachable using the Toolbar icon).
Updated icon.

Added Pop-up button functionality. Now users can edit their custom CSS using the Add-On's Pop-up as well as within the Options page.