If you have a user with strong HTML and CSS knowledge who is able to make reasonable changes to the entire layout of the website or just to the module templates, then you can assign that user to the group Designer.
A designer must have the following set of permissions:
This set of permissions allows the designer of the website to perform any task related to the design of the template. However, the designer will not be able to edit the content, submit news, or set any site preferences.
The designer still needs a content page that he can use for design or layout tests. In order to prevent the test page being shown in the menu or in the sitemap, you should do the following trick.