How can I change Highlight Colour + Highlighted Text Colour? (No plugins please)

Custom CSS & Theme Design
5

Those still look good holroy :+1:

/* ==highlight== background */
body {
    --text-highlight-bg-rgb: 255, 208, 0;
    --text-highlight-bg: rgba(var(--text-highlight-bg-rgb), 0.4);
}

/* ==highlight== text color */
.cm-s-obsidian span.cm-highlight, .markdown-rendered mark {
    color: blue;
}

You can separate these out by light and dark theme mode if need be, and the --text-highlight-bg: can be replaced with a color, rgb, hex, etc. e.g.

.theme-dark {
    --text-highlight-bg: orange;
}

but I would change the --text-highlight-bg-rgb: 255, 208, 0; values to get the alpha/opacity option in the second --text-highlight-bg property.