2024.04.16

Place this test file in the AutoIt folder where the AutoIt.chm is at.

Make a file called AutoIt.css with the CSS in it or with an import line, like:
@import "file://C:/Users/<myUsername>/AppData/Local/AutoIt v3/<don't know yet>/Buuf.css";

The CSS loading order is:
First try the SciTE's folder.
If that fails try the AutoIt's folder.
If that also fails then use the default.css from within the chm itself.

On WinXP, no external CSS will be loaded. 
The Emulation has to be IE11 for this to work.
Win7 onwards shows to work just fine.

In case of questions go to https://www.autoitscript.com/forum/index.php?showtopic=211676

Tested on Win XP,7,10,11