feature request: preserve syntax highlight on Diff* highlight groups

[horriblename]

it would be nice to have a darker background and preserving syntax highlighting on DiffAdd and DiffChange (and possibly others)

example (I just picked a random bg color):

for comparison, what it looks like currently:

it really helps with reading code, and doesn't assault my eyes like light mode does hehe

[ImmanuelHaffner]

I had to do the same for the diff colors. This is what i have currently:

hl(0, 'DiffAdd', { fg = 'NONE', bg = '#0f2909', })  -- bg a dim green
hl(0, 'DiffDelete', { fg = 'NONE', bg = '#331c1c', })  -- bg a dim red
hl(0, 'DiffChange', { fg = 'NONE', bg = '#181800', })  -- bg a dim yellow
hl(0, 'DiffText', { fg = 'NONE', bg = '#282800', })  -- bg a brighter yellow

I will happily file a PR with added highlight group definitions if we can agree on some nice colors :)