Icons of Navbar(?) not visible

[RoestVrijStaal]

Good evening,

I'm addressing an issue which I ran into for months, but today I was enough fed up of it to start digging in it.

The icons of the buttons westwards at the urlbar are not visible, and the "Downloads", "History" and "Bookmarks" buttons look "wrecked".

Take a look at the screenshots, made with a brandnew Pale Moon profile with only Dark Moon installed.

However, those icons appear when the menu-bar is disabled in favour of the big title button.

The bug seems to be Windows-only, since the problem isn't present on my Linux machine running Kubuntu 18.04.2 using Dark Breeze theme.

Used software in screenshot: Pale Moon 28.6.0.1 with Dark Moon 2.3.1