Open SharePoint 2010 site using browser.
Click on Site Actions and then Site Settings. This will open the site settings page.
From the Site Settings page Navigate to the Galleries section and click on Themes.
This will open the Theme home page.
From the ribbon, click the Documents tab and then click Upload Document. And browse the .thmx file that we have save in a physical location as described in this article. Also you can upload from the Add new item link presented below the list of Themes.
This will open the upload new item dialog box.
After that click on Save and our custom theme is saved and ready to use.
To use the Theme click Site Actions -> Site Settings. Then, under Look and Feel, click Site Theme. From the list of themes you can preview or use our custom theme.