Themes are predefined by Calc and easy to apply. The themes button is located on the Toolbar.
This is what the Theme icon looks like:
Here is the Theme window, there are a lot of choices.
Click on a theme and the sheet is changed to reflect the changes as a preview. Notice in the example that the theme will not force changes on row color that you have already made for example.
Changing back to the Standard theme will undo all of the changes.