The following are some Codium Extensions that I like:

Beautify
Borealis Theme (install via .vsix file on vscodium)
Bracket Pair Colorizer
Cobalt2 Theme Official (blue borealis)
EsLint
GlassIt Linux (or GlassIt depending)
GlassIt Note: You'll need to edit the extensions.js file for it to work with VSCodium:
If you are using VS Codium, you need to edit the extension.js file in the extension folder: ~/.vscode-oss(OR .vscode)/extensions/s-nlf-fh.glassit-{THE VERSION}-universal/, and change "code" on line 46 to "codium".
Import Cost
Markdown Preview Enhanced
Prettier
Vetur
Vim