I have added css for a website.

[CorruptPriest]

Hey, I have forked this repository and added CSS for mymind.com - Idk how to ensure it is working with the mod.
Here is the code so you or anyone else can add it -

html,
body {
background: transparent !important;
margin: 0;
padding: 0;
color: #fff;
/* Keeping the text legible on a see-through backdrop /
text-shadow: 1px 1px 2px rgba(0, 0, 0, 0.8);
/ A subtle shadow for better readability */
}

[sameerasw]

Sorry for the late reply, so the structure is basically the comments separate each feature so there can not be comments between any { }.
Second on is that you can't have hardcoded colors like #fff because that's gonna break in either the light or the dark theme so we better use light-dark(#fff, #000) like properties. And for this addon to work, every css property should be !important.
These are some basic guidelines I also mentioned in the readme and you can refer to existing themes as well :)