User blog comment:DaNASCAT/FANDOM's Mobile Experience Always Improving/@comment-29704864-20170726150810/@comment-27345308-20170726151758

I believe somebody mentioned they don't. There are much better ways for allowing customization than allowing users to edit whatever they want in a site's CSS. Nanaki made a thread for suggesting new features for ThemeDesigner back when the new header was announced, and if Wikia is going to allow any customizations to Mercury I'm pretty sure they will be through ThemeDesigner, so you should probably suggest new ThemeDesigner features if you want anything to change in the aspect of wiki customization.