Appendix table header is missing a background

[gustavi]

The URL : https://choosealicense.com/appendix/

The bug :

[mlinksva]

Only pertinent when scrolling beyond initially visible portion of table, introduced in #1089. I noticed this then, and decided it is a feature rather than a bug; no reason to obscure info that'd otherwise be visible. I guess if there's some way to giving the header a semi-opaque background so that it is clear it is not a bug, I'd be open to doing that.

[matthijsgroen]

Please make the sticky header more readable.