The default colorscheme is a mixed bag.
On one hand, you have a quite reasonable syntax highlighting and highlight groups that mostly use the so-called ANSI colors of the underlying terminal emulator, which are all good things.
On the other hand, the highlighting of UI elements is clearly a second thought, a light background is assumed by default, the highlight group are inconsistent, and Vim's own heuristics for determining what features are supported by the underlying terminal emulator are beyond broken.
Some of those issues could be easily fixable at a high level if the default colorscheme was actually a colorscheme like blue or desert but it is not so, as we focused our efforts on the latter, we didn't try to fix default.
It is on our radar, though.