If your curious where the default .less file that the brand file overrides it is here:
Finally found where this .less file exists. Use JustDecompile and open Microsoft.ReportingServices.Portal.Services.dll and find the Microsoft.ReportingServices.Portal.Services.SystemResources.UniversalBrand.less file so you can unravel the mystery.
Unfortunately a current version of the out of box UI .css is no where to be found. This needs to be remedied by Microsoft.
The ones indicated in this posting are for SSRS 2016. However there were alterations to this that Microsoft has not provided the defaults for in Power BI Report Server.
Update 1/4/2018: I was able to now just find the default colors file that you will have to manually reverse engineer back to a colors.json file in Microsoft.ReportingServices.Portal.Web.dll. The file in question is Microsoft.ReportingServices.Portal.Web.wwwroot.assets.css.brand_PBIRS.css. You would then need to take the .less file and map back all the colors to the LESS variables.
What Microsoft really needs to do here is provide their out of the box variables.less file, rather then just give us the compiled version.
I was able to cobble together a colors.json file based off of the LESS and cross referencing the resulting .css file output and mapping it to the LESS variables to get *most* of the default Power BI Server default color theme.
It is a bit different then what Microsoft has posted for SSRS 2016.
This took a lot of time to reverse engineer. Hopefully MS can fill in the gaps here, as it really needs improvement on their part.
I could not find the original definition for the theme>datapoints section or the following:
All others I was able to map back however. You can do a diff on this one and compare it to the out of box Microsoft one for SSRS 2016 to see all the differences.
Unfortunately after trying for hours to properly brand the Power BI Report Server I have given up.
Microsoft has made some mistakes with the way they utilized the variables in Power BI Report Server that were not made in SSRS 2016 such that it is almost impossible to get a good UI scheme working easily.
To give you some examples:
1. They used neutralPrimary on a large number of LESS variables that don't even relate to each other. So when you change that one it cascades down to a bunch of elements you don't want it to cascade down to.
2. They defined bodyBg in their LESS file to be neutralPrimaryAlt, but they also assigned it to toolbarPageBg!?