tests.plasma5: Fixes OCR in sddm by providing a custom theme

[bkchr]

The custom theme just sets the old background colour that OCR works
again.

Motivation for this change

Fixes: #42315

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[worldofpeace]

Tested it, makes the test do the thing again in the most logical way. 👍

[matthewbauer]

@GrahamcOfBorg test plasma5

[GrahamcOfBorg]

Success on x86_64-linux (full log)

Attempted: tests.plasma5

Partial log (click to expand)

machine: exit status 1
syncing
machine: running command: sync
machine: exit status 0
test script finished in 119.12s
cleaning up
killing machine (pid 593)
vde_switch: EOF on stdin, cleaning up and exiting
vde_switch: Could not remove ctl dir '/build/vde1.ctl': Directory not empty
/nix/store/vmynkx6wbnbg0jss9sx3f6fpmwyq5msk-vm-test-run-plasma5

[GrahamcOfBorg]

Success on aarch64-linux (full log)

Attempted: tests.plasma5

Partial log (click to expand)

machine: exit status 1
syncing
machine: running command: sync
machine: exit status 0
test script finished in 401.64s
cleaning up
killing machine (pid 627)
vde_switch: EOF on stdin, cleaning up and exiting
vde_switch: Could not remove ctl dir '/build/vde1.ctl': Directory not empty
/nix/store/awifdndpdxfaylw5kvaya1z282y61p86-vm-test-run-plasma5