There is not a good built-in way to do this. Although you can switch Firefox from using the colors specified in a page to colors you choose, this unfortunately makes many sites harder to use because they use color to indicate buttons or other important interface elements.

You might need an add-on. You could search here: https://addons.mozilla.org/firefox/

These look interesting but I haven't tried either of them myself:

• Color Transform (page color theme changer): https://addons.mozilla.org/firefox/addon/color-transform/

• Colorific (site-by-site page color theme changer): https://addons.mozilla.org/firefox/addon/colorific-1/

You can use the NoSquint extension to set font size (text/page zoom) and text color on web pages.

• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/

You need to restore this toolbar with an extension like this:

• Classic Theme Restorer: https://addons.mozilla.org/firefox/addon/classicthemerestorer/

• https://support.mozilla.org/kb/what-happened-to-the-add-on-bar