What I look for in a theme is:
1. Being easy on the eyes. 2. Easy distinction between different keywords etc. 3. Good contrast.
The best light theme I've seen so far is Selenized for VSCodium [1] and for VS Code [2]. There's a Selenized light for JetBrains IDEs too. [3]
[1] https://open-vsx.org/extension/santoso-wijaya/helios-selene
[2] https://marketplace.visualstudio.com/items?itemName=santoso-...
[3] https://plugins.jetbrains.com/plugin/23800-selenized-theme