Dual-Boot (Windows sadly) not showing in Grub & SDDM Theme help

Issue 1:
I have 2 SSDs. 1 has Windows (sadly I need it still because Unity Engine is not behaving on Linux), and the other Bazzite. I cannot see an option for Windows in GRUB. How do I solve this? I tried what I knew how to do from Arch and got nowhere. The workaround for now is to manually select Windows via BIOS.

Issue 2:
https://github.com/catppuccin/sddm

How do I install this considering that /usr is read-only (the theme goes into /usr/share/sddm/themes on traditional distros)?
GitHub
🔒 Soothing pastel theme for SDDM. Contribute to catppuccin/sddm development by creating an account on GitHub.
Solution
Looks like it's possible to specify an arbitrary directory for the themes already with SDDM: https://github.com/sddm/sddm/pull/1739#issuecomment-1914118894

As for installation of the dependencies and such, can probably reference this (though note the caveats): https://docs.bazzite.gg/Installing_and_Managing_Software/rpm-ostree/
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