Various dark theme fixes

[silverwind]

• beautify simplemde, remove header hover effect (does not play well with border colors), remove double border separators
• fix colored outline buttons
• fix disabled style
• fix positive messages
• fix markdown header border and quote block
• adjust footer color
• adjust file icons and release list timeline

Some screenshots:

[codecov-io]

Codecov Report

Merging #10416 into master will decrease coverage by 0.08%.
The diff coverage is 2.91%.

@@            Coverage Diff             @@
##           master   #10416      +/-   ##
==========================================
- Coverage   43.74%   43.65%   -0.09%     
==========================================
  Files         586      586              
  Lines       81265    81351      +86     
==========================================
- Hits        35547    35517      -30     
- Misses      41318    41441     +123     
+ Partials     4400     4393       -7

Impacted Files	Coverage Δ
routers/api/v1/repo/pull.go	34.82% <0%> (-0.36%)	⬇️
routers/repo/editor.go	26.09% <0%> (-0.66%)	⬇️
routers/utils/utils.go	72% <0%> (-28%)	⬇️
services/pull/merge.go	49.45% <0%> (-0.99%)	⬇️
models/error.go	29.49% <0%> (-1.46%)	⬇️
routers/repo/pull.go	28.61% <0%> (-0.09%)	⬇️
modules/repofiles/temp_repo.go	58.54% <13.63%> (-3.57%)	⬇️
services/pull/check.go	35.36% <0%> (-14.64%)	⬇️
models/unit.go	37.03% <0%> (-2.47%)	⬇️
modules/queue/unique_queue_disk_channel.go	53.84% <0%> (-1.93%)	⬇️
... and 6 more

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