I located the lines responsible and while I'm not sure if my quick fix breaks something else it's a starting point, I guess.
(...)
If (ShowPreview=0 || PrefOpen=0)
; Gui, OSD: Font, c%OSDtextColor% Bold, %FontName%, -wrap ; FIX: Remove "Bold"
Gui, OSD: Font, c%OSDtextColor%, %FontName%, -wrap
Else
Gui, OSD: Font, c%OSDtextColor%, -wrap
(...)
If (PrefOpen!=1)
{
FontSizeHorizProgress := (FontSize<60) ? Round(FontSize/5.5) : Round(FontSize/10)
If (FontSize<20)
FontSizeHorizProgress := Round(FontSize/2.9)
If (FontSize>130)
FontSizeHorizProgress := Round(FontSize/15)
If (FontSizeHorizProgress<9)
FontSizeHorizProgress := 8
; Gui, OSD: Font, s%FontSizeHorizProgress%, Arial ; FIX: Switch "Arial" to user-set FontName
Gui, OSD: Font, s%FontSizeHorizProgress%, %FontName%
Gui, OSD: Add, Text, x+0 -wrap vTextNavPrev, ----------------------
GuiControl, OSD: Move, TextNavPrev, h%smallLEDheight%
GuiControl, OSD:, TextNavPrev,
}
There's a third Font definition using Arial in this function, but I didn't touch it since changing these lines already solved the issue.