You can find by navigating to browser inspector (F12). There you search (CTRL+F to search) for something related to magento’s paths for example (catalog, skin, forntend etc.) then try finding where most on the skin css and js files are coming from for example: skin/frontend/COMPANY_NAME/THEME_NAME/. In most cases if the theme is not custom made, you will find it under the company name and the theme name. If you can’t find it, you can continue search some comments in the skin files in the theme js, css and etc.
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You can find by navigating to browser inspector (F12). There you search (CTRL+F to search) for something related to magento’s paths for example (catalog, skin, forntend etc.) then try finding where most on the skin css and js files are coming from for example: skin/frontend/COMPANY_NAME/THEME_NAME/. In most cases if the theme is not custom made, you will find it under the company name and the theme name. If you can’t find it, you can continue search some comments in the skin files in the theme js, css and etc.
If you have access to the site go to http://yoursite.com/admin
System > Configuration > General > Design > PackagesSystem > Configuration > General > Design > ThemesOr as already answered:
https://stackoverflow.com/a/32627148/3650708