Night Shadow Borderless by kittaro

Original instructions (please install css files from original version to full experience):

Firefox’s user interface is built of web-like elements (HTML and XUL elements) and styled using CSS. Thus, there are additional options for changing the appearance (user interface) of Firefox browser by applying style rules with an optional files named userChrome.css and userContent.css. With these files, the Night Shadow theme's color scheme is applied to all elements of the Firefox interface, including: internal pages (all about: pages), dialog windows, error pages, infobars, developer tools etc.

Download and installation instructions:
1. Install the core theme files on the "Add-ons for Firefox" web portal (addons.mozilla.org) by clicking the "Install Theme" button

2. Find your Firefox profile directory:
- Type about:support in the address bar and press "Enter"
- Go to "Profile Directory (Linux)" / "Profile Folder (Windows)" entry and click on "Open Directory (Linux)" / "Open Folder (Windows)" button

3. Download additional theme files (v20240125.01, 2024-01-25), unzip the downloaded archive and copy the "chrome" directory to the previously opened Firefox profile directory: https://github.com/serhiyguryev/night-shadow-theme/releases

4. Enable Loading of userChrome.css and userContent.css in Firefox:
- Type about:config in the address bar and press "Enter", click "Accept the Risk and Continue" to go to the about:config page
- Search for the toolkit.legacyUserProfileCustomizations.stylesheets preference and switch its value from false to true
- Restart Firefox


Attention: since the changes are mainly made in userChrome.css and userContent.css files, which are not updated automatically with the theme, you should check the theme's GitHub repository from time to time for updated versions of these files.
Original screenshots, download and installation instructions: https://github.com/serhiyguryev/night-shadow-theme

Big thanks for Serhiy Guryev for this amazing colors and tones mix