Add support for themes

[ExE-Boss]

Since Firefox 55, WebExtensions can also be themes that have some feature parity with Chrome and then some extra features on top of that.

The MDN documentation on the theme manifest key specifies that Firefox names its keys differently.

The Chrome documentation on the theme manifest key displays all keys supported by Google Chrome.

Transformation table

Chrome	Firefox equivalent	Notes
images	images
images/theme_frame	images/headerURL
colors	colors
colors/frame	colors/accentcolor	Firefox uses CSS colour codes whereas Chrome uses an array of RBG numbers.
colors/tab_text	colors/textcolor	Firefox uses CSS colour codes whereas Chrome uses an array of RBG numbers.
colors/toolbar	colors/toolbar (57+)	Firefox uses CSS colour codes whereas Chrome uses an array of RBG numbers.

[Noitidart]

Super super cool! Thanks for this information and this issue. Will definitely want this in the next version.

[Noitidart]

Planned for release in v3.5 - maybe 3.4