Zhiyi Zhang (@zhiyi) commented about dlls/uxtheme/draw.c:
glyph ? TMT_GLYPHTRANSPARENTCOLOR : TMT_TRANSPARENTCOLOR, transparentcolor))) { *transparentcolor = DEFAULT_TRANSPARENT_COLOR;
if (!has_default_transparent_colour)
*transparent = ALPHABLEND_NONE;
Let's extend this to the case when GetThemeColor() succeeds but the transparent color also happens to be DEFAULT_TRANSPARENT_COLOR.