Default master page of an SharePoint 2013 site is seattle.master In order to apply your custom theme to your SharePoint Site you need to either change the css of the default master page (Which is not at all recommended) , Or you can download a copy of your SharePoint master page and make changes to it. You can change the master page tags as well as css using either Visual studio or SharePoint designer as an editor. Let’s start with getting a local copy of SharePoint Master Page & its theme. 1. Open your SharePoint Site. > Go to Site Settings > Master Pages. (This will take you to master page catalog’s library, you can download a copy of master page from there). 2. Now let’s get the css. Open the below link where you will find the default css of Seattle master page. http://Your Site url/_layouts/15/1033/styles/Them able/corev15.css Get the css classes to your local machine. 3. Now we just n