stylix: refactor overlay to mirror mkTarget

[0xda157]

cc @MattSturgeon

should be merged after https://github.com/nix-community/stylix/pull/1384

Things done

• [ ] Tested locally
• [ ] Tested in testbed
• [x] Commit message follows commit convention
• [ ] Fits style guide
• [ ] Respects license of any existing code used

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