Custom Style Manager by Halil Can

Currently, Custom Style Manager allows you to make styling changes to the active domain through the editor available in the Firefox Toolbar. Your changes will be synced across all compatible logged in Firefox browsers.
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I made this add-on primarily because I wanted the ability to easily and permanently change the look and feel of websites whenever I might want to. If you have some CSS literacy and occasionally feel the same, I hope you will find it useful.
CSM is fully open source; if you are the kind of person who might find some use in it, then you might be the kind of person to improve it at https://github.com/HalilCan/customStyleManager.