Add Gradient Option for Custom Theming

[BenAlanSouthall]

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Is your feature request related to a problem? Please describe.
As per #19040 and #19669 it seems that the gradients were disabled for users applying a custom colour to their instance.

If the gradient were applied with the secondary NC blue, then this will probably look ugly by default, however, personally I think the gradient option looks really nice. It works well with the default blue and I think adds a subtle but really significant level of professionalism to the theming. A flat colour by contrast can cheapen the overall aesthetic for me.

The branding for our company for example uses two colours, and it doesn't look great being stuck to use one or the other in the theming of our instance.

Describe the solution you'd like
I'd like a simple way to specify the main theme colour, plus a secondary gradient/accent theme, in the admin settings, under the "Theming" page.

Describe alternatives you've considered
At the simplest option, I'd like a basic option to be able to set the left and right colours separately in the theming admin options. By default one would be greyed out or set as the same as the other, so this replicates the current functionality of using a flat colour by default. One would presumably be the primary colour for folder icons, etc., whilst the other would function only when gradients are used. It would be assumed ot be a 50% linear gradient.

More advanced configuration could involve the ability so set multiple colours and position gradient stops for these colours. This might provide room for havbing different accent colours later should users desire this. Alternatively setting an image for a headerbar could be a simpler way to achieve greater configurability. I am thinking a bit like how MS OWA and some other mail clients have a headerbar configurable with some rather poppy themes. Some organisations might also have logos that don't fit the standard icon aspect ratio so well. In fact the nextcloud site itself uses this to good effect. So this might be preferable, or would be a nice feature to have separately.

Additional context
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