A user style manager which can load local files and apply UI styles even in Firefox 57+
What is reStyle?
reStyle is a user style manager add-on for Firefox 57+ that supports installing styles from userstyles.org, GitHub, and other websites. It allows you to load local styles and apply UI styles to the browser. With reStyle, you can customize and update styles for normal websites, browser UI, and about:-pages. It also offers a development mode for loading local styles and requires NativeExt to be installed for enhanced functionality.
This user style manager supports the usual installation of styles from URL and polling those URLs for updates. In addition to that, you can load local styles and apply styles to the browser UI.
Styles can be installed from userstyles.org, GitHub or any other website that offers appropriate styles for download. On userstyles.org, open the styles detail page, choose your settings if the style offers any, then click the reStyle icon in the browsers toolbar and "Add style" in the popup. On other pages, you need to open the .css file before clicking the reStyle icon.
Starting with Firefox 57, it is no longer possible for Add-ons to directly apply user styles to anything else than normal websites. As a workaround, reStyle recognizes styles that will no longer work and writes them to the
reStyle is risky to use as it requires a number of sensitive permissions that can potentially harm your browser and steal your data. Exercise caution when installing this add-on. Review carefully before installing. We recommend that you only install reStyle if you trust the publisher.
reStyle has earned a fairly good reputation and likely can be trusted.