Use new assistant theme

[julien-nc]

closes #347

• Adjust header menu icon: use NcHeaderButton (that comes with a transparency gradient)

• Adjust the main form title (icon + text color)
• Change selected task type category style

If we want to keep supporting NC 30 and 31, we need to be careful with this custom style that is not included in NC < 32. There is now a use-modern-style initial state injected to decide if we use the new css vars or not.

@marcoambrosini We can't use NcAssistantButton because

• NcActions is not flexible enough
• Even if we could use it to replace our NcButton in some places, this component does not allow to use a custom icon so we're stuck with the assistant one but we want the category one

So I went with custom styling using the css variables.
I tried to replicate the gradient in the NcAssistantButton component but this is done by using a gradient background in an element wrapping the NcButton. Here we directly use the button, no wrapping element. I couldn't find a way to have this gradient border so I removed the border which IMHO looks better than the border gradient + a different button gradient.
This is how NcAssistantButton look like with the 2 gradients:

[marcoambrosini]

Would it be possible to at least add a solid border for now?

[julien-nc]

With classic border and color being #40519A:

[marcelklehr]

🎉