Some new options were indeed added for KelticDanor's recent skins. They will be documented at the next XMPlay and skinning kit release, but in the meantime, I think this is all of them:
color_listnum = track number/time/type
color_listqueue = queued track marker/number
color_listselback = selected track background
font_listposalign = playlist position/count alignment
font_searchalign = search text alignment
infostretch = stretch the info window heading (instead of repeating)
pos_listoff can also now include a 2nd number for right side padding
The best way to find all of the other existing skinconfig options is to look at the example skins in the skinning kit, as all options should be listed in those (even if they're unused/commented-out). For example, pos_info1line and font_mainalign are included there. I notice the font "quality" options aren't though. They set the "iQuality" parameter in the font's CreateFont call, as described here:
https://learn.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-createfonta
