Custom color for cell numbers and markdown links

[amanjitsk]

Hi,

Thanks for the amazingly useful piece of software. I was wondering if it'd be possible to add support for customization
of the colors of cell numbers and other elements like markdown elements. These become very hard to read when
using a dark scheme (for example with a custom color scheme). Also, the mulitplexer passthrough has weird behavior
with colors when set to True and used within Tmux, but I am happy to open another issue detailing that as well.

Best,
Amanjit