Theme"JarvisChan" follows your system's light or dark mode.
Applied on top of the theme, live. Whole imageboard themes can be pasted as-is: the variables (--post-color, --link-color…) and classes (.post-container, a.quote, .catalog-tile…) are the same.
Some themes point at images on their own server, like url(/css/themes/assets/iwakura-bg.gif), which don't exist here. Upload the image and it replaces those links. Saved in this browser only.