To reproduce follow instructions below:

• Set Qt Creator Dark theme of Text Editor.
• Create .cmake script.
• Type set(@_. All text should be visible by now.
• Type any letter. The part starting with underscore symbol visually disappears due to foreground color becoming equal or very similar to background one.

There seems no way to change color of such text in Settings. Looks like some hardcoded color is used.

This issue affects writing template files for configure_file command.