Use configured background color during initialization

[jpovixwm]

When using the dark theme, this will shorten the user's exposition to the eye-blinding white background during startup. If custom color overrides were set up using a prior version of TrguiNG, the user will need to re-apply their custom colors, because otherwise the computed property will not be present. I don't think it's worth it to try and handle this edge case, so I left it as is.

[qu1ck]

Current implementation is fine, thanks. Mantine moved away from emotion-like styling and when TrguiNG updates to mantine v7 styling needs to be reworked all over and this will likely be updated too.