The theme that is shipped out-of-the-box is called Default. You can export the default theme and use it so that you can start quickly. You can customize the appearance and behavior, which includes the layout by modifying the .css file, import and apply this new theme, and then you can use the customized appearance and behavior. Using the .css file also makes it easier to work with your web designers.
The following cmdlet creates a custom web theme, which duplicates the default web theme.
New-AdfsWebTheme –Name custom –SourceName default
You can modify the .css file and configure the new web theme by using the new .css file. To export a web theme, use the following cmdlet.