Theme updates for Nova

[rebloor]

This change updates the guidance on creating themes to include:

• considerations when creating themes for Nova. (WIP, pending further information.)
• information on using gradients.
• information on the placement of gradients and images in the header or across the whole UI.
• updates, including new Nova-themed screenshots for the pre-existing guidance on creating themes with images.

[rebloor]

@bacharakis I've reviewed and removed the to-do notes and checked the content. To-do items were:

• details on restrictions regarding accent color. I updated this to match the description in the draft blog post
• describe how a theme_frame image behaves behind the toolbar and the sidebar in Nova. Cover how the image aligns when the sidebar is open, and whether any part of the image is obscured or stretched. This came from a comment on the Jira ticket, but I had no details. Hopefully, the (known and documented) information gradients and images included cover this.
• Document the known sidebar limitations, including the hover states in sidebar panels and the behavior with expand-on-hover enabled. Note which keys have no effect in these cases. Again, from the Jita ticket with no details. Keys affecting the sidebar are documented, but I'm not sure whether the information is consistent with the blog post comment "Sidebar buttons follow your text color. Buttons and icons in the sidebar are derived from your text color rather than your toolbar button colors, so they can look slightly different from the toolbar."