Yes. Themes and custom CSS are built into the web and desktop apps for everyone, with or without Plutonium, and you can share themes with others. The apps article says which apps have which theme settings, and third-party mods are allowed on Fluxer too.
Are themes and custom CSS allowed on Fluxer?
Yes. You can change how the app looks with "Quick CSS" or saved themes under "Themes" in "User settings", and you can import a theme someone else made.
Is it safe to install a theme?
A theme can change anything you see in the app, including hiding or relabeling buttons and messages. Install themes only from people you trust, and look through the CSS first if you can. Some themes load fonts or images from other websites, and each of those sites sees your IP address when it loads, like any website you visit.
A theme broke the app. How do I turn it off?
Press Ctrl+Alt+Shift+K, or Cmd+Option+Shift+K on macOS, to turn custom CSS off. That's the default keybind, and you can change it under "Keybinds" in "User settings". In the desktop app you can also choose "Turn off custom CSS" in the tray menu, and in the web app you can add ?custom_css=off to the end of the address and load the page.
Custom CSS stays off in that tab or window until you turn it back on or close it, which gives you time to fix or remove the theme that broke the app. Here's how to fix the CSS that broke the app and how to turn custom CSS back on when you're done.