The theme name should be present in the source code of the page unless the admin deliberately changes the name.
Here are the steps to know the theme name:
- Open the website in a browser.
- Right-click on the page and select "View Page Source".
- In the source code, search for the word "stylesheet". The first occurrence will be core CSS, and the next should be theme CSS. In the href value, you can see the name of the theme.
Here is an example. The website is using oceanwp theme.
<link rel='stylesheet' id='font-awesome-css' href='https ://qzstatic.bitsdiscover.com/wp-content/themes/oceanwp/assets/fonts/fontawesome/css/all.min.css?ver=5.15.1' media='all' />