This Firefox extension allows you to toggle css styles on the fly
Make sure you set up your userChrome.css and can add css styles to your Firefox. (Guides: reddit, userchrome.org)
Now, to use toggleable styles you need to copy the selector of one of the six available toggles. To do this, click on the button labelled "selector". When you click with the left mouse button, the required text is copied to the clipboard. Next, paste the selector into your userChrome.css, add curly braces. All styles inside these brackets will be applied only when the toggle whose selector you copied is active.
Example:
Copy 1st toggle selector *click*:
Paste it into userChrome.css
and add brackets:
Write some styles inside:
Toggle:
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Example of multiple styles:
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How to customize shortcuts: