Click on Profile on the menu bar.
A new menu bar appears underneath. Click on Modify profile and select Looks and layout.
The curent scheme is called Fresh Looks. At the time of writing it is green and red. If you want to change this click on change.
A drop down menu appears which allows you to change the colour.
The green stays - it's the red that changes to orange, blue, black or green.
You see below two other blue schemes that look similar. You can change to these too but there is a warning that Core theme doesn't work well on some browsers.
Experiment and see what you like.
Further down the page you may almost miss a series of check boxes.
If you don't like horizontal menus there is a option to change them to appear down the left hand side.
You can return to the others at a later date
Save your settings by clicking on change profile.