The widgets are now fully customizable via Style Sheets.
https://github.com/LibreCAD/LibreCAD/wiki/Style-SheetsHowever, icons are a different issue.
I'm hoping we can replace the current PNG icons
with a SVG set (artists wanted).
Regarding custom icons:
I'm more inclined toward allowing users to
select a folder that contains a replacement set of icons,
as opposed to using pixel manipulation.
Users could open the SVG files in Inkscape for editing;
and we could change the colors and gradients for every icon
using something built into LibreCAD, or independent scripts.