Problem:
Things like underlines are always given a default foreground highlight
regardless of the value of `sp`.
Solution:
Check for `sp` first, and apply that color to the text decoration color if it
exists.
Limitations:
If there is no value of `sp`, vim applies a text decoration color that matches
the foreground of the text. This is still not implemented (and seems like a much
more complex problem): in TOhtml, the underline will still be given a default
foreground highlight.
59baa5e8a1
