Click on add new and then go to the search bar and type some keywords you want to look up as a theme. Sometimes themes will show up and sometimes they won't depending on the keywords you type in. This is not doing a keyword search, it's just merely trying to locate a theme to possibly suite your vision and niche.

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Then simply scroll down and look through the themes that appear. Just hover over the theme and click on Details and preview and it gives you just that.

Look for one you like. You can possibly search for one that is mobile friendly as well. I like for my site to look good everywhere.