I was wondering if there is any interest in background themes for a youtube channel ? If so, on which marketplace can I present such backgrounds and in which catagory ?

Hmm, I would say it’s a GraphicRiver file but I don’t know which category…
- Microlancer Beta Tester
- Sold between 50 000 and 100 000 dollars
- Exclusive Author
- Bought between 100 and 499 items
- Referred between 500 and 999 users
- Author had a Free File of the Month
- Author was Featured
- Item was Featured
- Europe
I think you can make YT wallpapers but you can’t use any brand name such as YT. You can place it here as JPG or PSD :
http://graphicriver.net/category/graphics
Also think about Twitter backgrounds 
- Beta Tester
- Bought between 50 and 99 items
- Contributed a Blog Post
- Exclusive Author
- Has been a member for 3-4 years
- Interviewed on the Envato Notes blog
- Item was Featured
- Netherlands
- Referred between 1 and 9 users
This may belong to the category ‘Backgrounds’, since it isn’t allowed to mention ‘YouTube’ in your file. Otherwise I would choose for the category ‘Graphics’, indeed. Maybe are Twitter backgrounds a bit more popular, but remember that there are already many backgrounds for Twitter at GraphicRiver. I think this could be a great idea, good luck! 
