#create new theme named centeredUI
New-AdfsWebTheme -Name "centeredUI" -SourceName default

#replace default stylesheet with new sign in experience CSS
Set-AdfsWebTheme -TargetName "centeredUI" -StyleSheet @{path="C:\temp\centeredUI\ThemeCenterBrand.css"}

#replace default onload.js with paginatedonload.js
Set-AdfsWebTheme -TargetName "centeredUI" -AdditionalFileResource @{uri="/adfs/portal/script/onload.js";path="C:\temp\centeredUI\paginatedOnload.js"}

#set new theme as active webtheme
Set-AdfsWebConfig -ActiveThemeName "centeredUI"