You can modify some graphic elements by going to Configuration > Customize design.
This page is organised by your website area: Page, Site, Header, Content, Menus, Footer. To vizualise the area you are editing, go to the link « place this area ».
For each area, you can modify the colours (text, background, link), the image background, borders, type characters, etc.
After each modification, validate the form at the bottom of the page by clicking Save then go on your website to check the modification.
Note the coherence graph on your website. If you are uncomfortable with graphic editing of your site, it is better not change the theme and keep a clean website which is consistent graphically.