Your website should show your brand, which is much more than your logo and name. You can use a theme to show some of it, if your brand is modern and cutting edge you would not use the same font as a traditional and old fashioned brand.
Some themes are flexible and others are how they come with no changes. There is a feature filter that you can use to find themes that allow you to filter out themes that you are searching for by subject or more importantly features like Custom Header, Custom Logo, Custom Colours and more.
Then use the Search themes to put in keywords. Find a theme that is or can be made to fit the brand of your business. If you apply one and do not feel it fits, simply apply another one and delete the themes you do not want to use.
If you have used text styles then applying a theme will update your whole site quickly and let you see whether it the right theme for you!