This sets a "flat" status line, vert bar and tab bar in all variants.
It does not have any effect for solarized8_flat, because "normal" and
"flat" are effectively the same in that case.
The separator makes it clear how a window has been split, which is
useful, for example, when you rotate windows (<c-w><c-r>). The separator
may be suppressed by the user with 'fillchars'.
See also: https://github.com/vim/vim/issues/2724.
Merge dark and light variants together. Now, you must select the
background color before loading the colorscheme, e.g.:
se background=dark
colorscheme solarized8