Add theme gradient based on primary and secondary colors

[JohnPixle]

Now that we have the primary and secondary accent, we can register our own gradient presets, in order to use them in the layouts.

---

Figma

[mghenciu]

I started playing a bit with the Gradients in Figma:

it may be needed to increase the contrast between Primary and Secondary accent, to have a more visible gradient. Right now it may be hard to see the difference between those two colors.

Or we can add another color, that is complementary to the Primary color - and use that when forming the gradient.

Let me know your thoughts, John - when you have some time.