To add the darkmode to your FS25 dediserver webinterface you have to add the following lines at the top of your css files in your installation.

/web_data/grid.css

@import url("https://cdn.jsdelivr.net/gh/yellowfromseegg/FS25-Webinterface-DarkMode@main/dark-theme-grid.css");

/web_data/main.css

@import url("https://cdn.jsdelivr.net/gh/yellowfromseegg/FS25-Webinterface-DarkMode@main/dark-theme-main.css");

after that, reload the site and have fun with the darkmode.