The question about checkboxes inspired me to try to change the colour of unchecked checkboxes in Clear Recent History (ctl+shft+del) in History > Clear Recent History.

I found the menu command in the Browser Toolbox and I used that to identify the modal. On my system is was the 343rd menuitem. My code doesn’t work but I don’t know why.

#menu-history-clear-recent-history{ .checkbox-check { appearance: none !important; background: #e2cfb6; } }