Add UI option to configure an extra logo variant

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Is your feature request related to a problem? Please describe.
It has been previously discussed in nextcloud/richdocuments#2832 that, when using a custom logo which is white or black, it can be difficult to see when using Collabora. We have already added a workaround for this in nextcloud/richdocuments#3840 by allowing to set CSS variables using the custom theming app to control the logo background color or visibility, but this is simply a workaround.

Describe the solution you'd like
A proper solution would be to allow the user to set both a light and dark variant of their custom logo to be used when the logo would be otherwise difficult to see (for example, when using a white logo and opening Collabora in bright mode).

Describe alternatives you've considered
The alternatives considered are mentioned above with nextcloud/richdocuments#3840, but it's simply a workaround and doesn't provide a clear way to solve the issue. It would be better to present the option in the theming settings by default.

Start by reviewing the theming settings entry point and the workaround discussed in nextcloud/richdocuments#3840, then compare the prior discussion in nextcloud/richdocuments#2832. Done means users can configure both light and dark custom logo variants and the appropriate variant is shown when Collabora's bright or dark mode would make the logo hard to see.