First, click on Profile menu and select Profile option (see attachment 1)
Then click on Modify Profile and select “Look and Layout” option (see attachment 2)
Click on “change” link next to current theme (see attachment 3)
Then simply choose from the available themes by clicking “Use this theme”.
More themes may be added in the future. The current choices are a mix of dark vs. light backgrounds and responsive vs. non-responsive (non-responsive does not re-arrange screen sections to fit different form factors)
Keep in mind that different themes may move things around from what you’re used to so it may take getting used to things and exploring a bit to find what works best for you.