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.
The only other things that differ between his theme and default Aero ( except the glyphs) about the caption buttons that I can see are:
1) He imported Win 7 caption button images
2) He changed the content margins values on the button images so that the glyph glows would fit correctly in them and also the sizing margins values on the close button
Take a look at this post:
http://vistastylebuilder.com/forum/index.php?topic=50.msg197#msg197Using large negative values for the bottom values of the content margins of his caption buttons ( such as -9 on the close button) forces the 192dpi glyph to be used. So there's your answer