For my personal use I will change html too, but to share, it will be just css, this will avoid any problem or incompatibility. 
But I noticed that the css of the page “Design” has no effect on all areas of Cyberpanel, so they will be updated directly via sh.
This allows in the future, you add a theme selector in addition to the common css page (/base/design), as it currently works. I will plan the best option and then share here for analysis. For now I’m just playing with styles for personal use. =)
If you could make the css of the customization page have an effect on all areas of Cyberpanel, this would solve a full customization. Can you do that for the next update?