The next version will have bootstrap5 support, and bootstrap5 has support for a dark theme. It's not quite ready in SeedDMS, but it's just a matter of time.
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If anyone is using Stylus and interested in a kind of dark theme for SeedDMS
https://sourceforge.net/projects/seeddms-manual/files/Stylus-Themes/
Tested with SeedDMS v 6.0.41 and Firefox.
The next version will have bootstrap5 support, and bootstrap5 has support for a dark theme. It's not quite ready in SeedDMS, but it's just a matter of time.