Custom themes are always a joy to see on websites and apps, which is why today I am suggesting a system for them. You can change the main background color (defaults: white on light theme or black on dark theme), and the highlights (defaults: purple on light theme and orange on dark theme) to whatever colors you want. In the case of the background, you could change that to an image. This leads me to my second point: More mod page customization. You could allow for the use of custom fonts, custom color schemes, backgrounds, more formatting, and all that other good customization stuff. A user could have the option to override a mod's custom page design with an option, getting a notification that you may not be seeing the author's intended experience.