This add-on give you the ability to change your theme for one of our preset themes inspired in the GX style, compatible with light and dark color schemes; this add-on give you too the ability to change the 'Colors' of your browser to make it all yours, you will be able to change:

(Frame) The background color of your tabs.
(Selected tab) The background color of your selected tab.
(Toolbar) The background color of the toolbar.
(Accent color) The color that give your selected tab a stronger/gentle border color to distinguish it.
(Top separator) The separator line between tabs and toolbar.
(Icons) The color of the icons so that they have good contrast.
(Urlbar) The color of the URL field.
(Panels) The background color of the arrow-panels (popup panel).
(Button hover) The color for your buttons when you :hover on them.
(Button active) The color for your buttons when you click on them.

You have too the possibility to change some font colors:

(Unselected tabs) The color of your unselected tabs.
(Selected tab) The color of your selected tab.
(Toolbar) The color of the toolbar (url) and bookmarks.
(Panels) The color of the letters into your arrow-panels (popup panels).
(New tab) The color of the letters of your NewTab page.
(Sidebar) The color of the letters of Firefox Sidebar.

REQUIRED for dark theme: Set * svg.context-properties.content.enabled * to true in about:config page to fix various icons colors of this extension. How to do this. Known Limitations

Partial support for theme with images, in some of them the image will disappear if you try to apply the change of colors.
If you use the 'system theme - automatic' you are going to experience various problems, to resolve it you must first choose another theme, other than the automatic one, on the 'about:addons' page.

For a complete customization you could try the theme ahead: Firefox-GX.