XnView Software

method to reproduce:
---go to Settings > Thumbnail > Labels tab
---add File name to right column, select it and adjust Text colour
---close settings and see thumbnails

result:
--colour of filename in "thumbnail + filename" view --- not changed
--colour of filename in "thumbnail + labels" view --- changed

hard-coded in my style_sheet file?

you must check File List>Item colours too

Ah, now I see! I'd never thought of using file types...

but I have to change the "default colour", the one at top right has no effect.