Installed Arduino IDE 1.8.5 from flatpack.
The display has some issues:
- The window scroll-bars'-handles are not visible: Scrolling works via the mouse just the handles are invisible.
- The messages' background is white - it supposed to be black. Under the displayed message the BG is black.

Otherwise the IDE compiles/uploads OK.

I'm guessing, this is to do with Look-and-feel.
I changed to other LnFs, no improvement.
Switched to different colour scheme - no change.
Tried to edit the colour scheme but didn't find anything related to window-sliders.
Tried to edit the 'Plasma theme::widget/scrollbar' but clicking on the 'Open in Editor' button didn't do anything. (Arc-dark, Fantasy-blue etc. )
The 'Breeze-dark, Oxygen etc. theme don't offer any editing of widgets/scrollbar. (Whatever this editing may be. Haven't seen it yet.)

The same version displays fine in Mint.

I am lost how to go about fixing this.