I previously reported that some of the bottom panels in PB don't use the Windows Control Panel "Window" background color. I found that I can edit notes.css, but wasted some time today specifying valid colors in formats not supported by PB4.
My suggestion is to add a few comments in the css files to remind users of PB4 / css limitations. (How often do you get a five minute to implement suggestion?)
In this case I discovered that only hex color or HTML name formats work in PB4 notes (and I didn't check the color names extenseively). That is:
background-color: Window; /* does not work */
background-color: RGB(192,220,192) /* does not work */
background-color: #C0DCC0; /* DOES work */
background-color: blue; /* DOES work */
I've added a comment to my notes.css file that might very well save other people the trouble of finding out that PB4 doesn't support the RGB or System-Defined System colors:
/* don't use RGB or User-Defined System colors */
It could be that there are other limitations in the .css file format due to (for example) your multi-platform implementation. If this is the case, the notes.css file could include a simple higher-level comment about which flavor of css is supported, as appropriate.