Add UI option to configure an extra logo variant

[elzody]

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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.

[juliushaertl]

Additional note, this can also be useful for other places like the email template where a logo might be rendered on white background, currently we workaround there by adding a small pill with the primary color around it.