As far as I can find, there's really no way to get this spread out to the whole space. This is because A) different browsers have different default font sizes... so the menu is bigger, and could cause the menu to drop to a second line, and B) as people increase their font size, different browsers round up differently for padding, so again, it difficult to get the menu to the edge without making it go over.
There's probably a little room to play with, to get less space left over, but I figured when designing it it was better to make it look like its *supposed* to have leftover space rather than getting close but not quite, KWIM?
If you want a larger font, you can make it a tad larger, then reduced the 2em left and right padding. (padding: .6em 2em .6em 2em;)
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