Split view diff with arc-green theme have white background

[relta-c]

• Gitea version (or commit ref): v1.9.0-rc2
• Git version: 2.16.4-lp151.3.4
• Operating system: openSUSE Leap 15.1
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes - here using arc-green
  • No
  • Not relevant
• Log gist:

Description

Split view diff with arc-green will display white background in added line.
I'm not really a web developer so I don't really know much about CSS and stuff
but after digging around a bit it seem to be a result from this line in arc-green.less:

.repository .diff-file-box .code-diff-split tbody tr.add-code td:nth-child(1),
.repository .diff-file-box .code-diff-split tbody tr.add-code td:nth-child(2),
.repository .diff-file-box .code-diff-split tbody tr.del-code td:nth-child(3), <<
.repository .diff-file-box .code-diff-split tbody tr.del-code td:nth-child(4),
.repository .diff-file-box .code-diff-split tbody tr.del-code td:nth-child(5),
.repository .diff-file-box .code-diff-split tbody tr.del-code td:nth-child(6) {
background-color: #2a2e3a;
}

Change it to
.repository .diff-file-box .code-diff-split tbody tr.add-code td:nth-child(3),
seem to fix the issue.

Screenshots

[silverwind]

Probably a regression from #7279.