Hi Gunnar - while driving to a job today i was thinking about that very topic. Should I make a video on the graphics themselves because they are obviously an important part of the whole thing. Im very much leaning towards doing one but its not really about the Ruleset Wizard so tossing it up still..
Anyway - to answer your question -
If you look at the original graphics for fieldlight and fielddark they dont really give much away. You have to look at where they are used and how. In CoreRPG and 5E both of them are used as frames to hold other elements and they are used to frame specific content like a number or a string.
So we have a couple of clues.
They are always going to be displayed as a square/rectangle.
They are displayed in both small (as small as 25x25px) and much larger elements - eg behind/around a list of Stats.
They have at least 2 versions - a darker one and a lighter one.
So pulling back out a bit and looking at the whole character sheet and then at the whole rulebook artwork I look for easy elements to reproduce that will give me a similar look and feel - I dont need identical, or even close, I just need the feel - and I start working with those.
When designing any of these graphics its important to remember the 9 slices thing.
Top Left is fixed
Top tiles horizontally
Top Right is fixed
Left tiles vertically
Middle tiles vertically and horizontally
Right tiles vertically
Bottom Left is fixed
Bottom tiles horizontally
Bottom Right is fixed
This means that gradients and decorative elements need to stay within their slices and if the slice tiles make sure the element tiles nicely too.
These rules apply regardless of the size of the element.
So elements like fielddark and fieldlight that are used with small sizes should be quite simple.
Elements like ChatWindow and Character Sheet and Combat Tracker can have more decorations.
I think that most people should stick with relatively simple frames - if you are graphic designer then get funkier - but only after you really understand how it should work.
I hope that helps?