To obtain, edit, and import a theme follow the steps below:
1. Open the Portal and go to Portal Tools > Manage Portal Framework > Theme Manager

2. Export the theme and save it to a location. Open the folder when the theme has saved to the folder. 

3. Edit the folder name from pwtheme to be a zip and select yes on the popup confirmation. 

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4. Extract the folder.
5. Edit the contents of the theme to your liking. You can alter the CSS, 'Actions' icon, customize the look of modules, and so more!
6. Rename the theme folder name and CSS (CSS file name has to be same as folder name) if you're adding a new theme. 

NOTE: If you're editing a default theme, you will want to make sure you rename the folder and CSS file names. If you do not, any changes made to the default themes will be overwritten in an OnSemble Platform update. 

7. Zip the contents of the folder and change the extension from .zip to .pwtheme.
8. Import the theme.

Make the theme available for selection if it's a new theme from the Theme Manager page. Then change your theme to the new one by going to My Settings > My Preferences > My Themes tab,  to make sure it looks good. 

Use this article to guide you through how to set a theme for all users.