Because of the way the coloured boxes were generated it was non-trivial to apply the user css
Let me clarify: the problem is that the CSS specifies the text colour (AKA foreground colour), while the boxes originally depended on setting the background colour. AFAIK there's no way to tell CSS "swap foreground and background colour". Hence, in a next phase, a completely filled block character was chosen, as quite a few people weren't happy with the spades. It seems an equally important section of our population feels that way about the blocks.