Depending on the conditions around where you normally use a computer, you may prefer an overall darker environment if you find that the bright white hurts your eyes, or a lighter environment if you’re in a well lit space and it’s hard to see the screen otherwise.
You can make either of these changes in the same place.
In the Global Theme window, you have 4 options available, 2 are various options for a lighter interface, and two are options for a darker interface. You can select one and click on Apply in the bottom right to try it out. It may take a moment for your computer to adjust.
There are other themes you can download and install by selecting Get New Global Themes...