"Never show close button" option breaks theme preview/editor
If I use a theme, be it imported or created, and check the checkbox for the option "Never show close button (option above will be ignored)" the theme preview/editor in the "Tabs look" section of the settings page breaks if i reload it.
Only the normal tab states are visible, as can be seen below.
The theme and everything else does still work as intended for the tab bar itself. The close button option does also have the intended effect.
If I uncheck the option and reload the settings page the theme preview/editor looks as it should again.
As the "Active" tab state is the first one that isn't shown correctly and that row is where the option for setting the close buttons background color, the last close button related theme option, I would suspect that that is related to the issue.
This is on version 1.5.3.
On the Linux version of Firefox 60.0b6 (64-bit).