I'm happy announce that I have just added a much requested feature to my plugin UI Customizer (UI Customizer) ... Themes!
There is 6 themes included at the moment but i'm planning to add more and the themes are going to be updated/handled without updating the plugin in a future version.
There is thumbs of the themes here : OctoPrint-UICustomizer/octoprint_uicustomizer/static/themes/thumbs at main · LazeMSS/OctoPrint-UICustomizer · GitHub
Let me know your ideas and comments.