`COLOUR_WHITE` was being passed as the colour when drawing
any text. The Options window uses the `rct_window::colours`
array for resolving the correct theme colour.
Replacing all occurrences of `COLOUR_WHITE` with
`w->colours[1]` makes the text colours more consistent.
In the details section values are still painted in black
no matter the colour theme, which is consistent with e.g.
the park information window.
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