I am currently working on a project where - while coding - there are some wishes about the layout (changing font color, changing the texts etc.)
What would be the best category in tfs?
bug, feature, issue or task?
I've read that: What are the differences among bugs, issues and tasks in TFS
But this did not help me on that question
To me, this would be either a Bug or a Feature.
It is a bug if it was supposed to be that way, and was not done.
It is a feature if it is a new idea/change.
After you create the bug/feature, you will add tasks to it as children. Those tasks should be the low level things needed for the overall feature to be done.
(By the way, if you are doing scrum, "features" are usually called Product Backlog Items. But both Bugs and Product Backlog Items go on the Product Backlog.)