Thanks for the explanation. Following your instructions, I tried importing my own glyphs. Since the 100% (96dpi) glyphs are too small for my TV, I used W8's display settings and set 150% (144dpi). But importing a new 144dpi glyph changed nothing. Even importing new 96dpi and 120dpi glyphs had no effect. But when changing the 192dpi glyphs, it finally worked. So, it seems that this theme is somehow fixed at 192dpi, regardless of the actual display settings.
Now, I wonder how this is achieved.