You can customize the look and feel of the Power BI reports using the formatting pane for each specific visual or you can speed up the process by using a JSON file.
This feature (the JSON file) has been available for quite a while but on the September 2017 update of Power BI Desktop, they allow us to define even more elements and that is great, but the documentation is poor (sorry Microsoft, it is) and unfortunately, a lot of the elements wont work even when you add them correctly to the file.
To be able to manage this, I have created my own guide for the JSON file and I am sharing it with all of you so you dont have to waste a lot of time finding out how to build it.
This is an iterative process and I am still working with the file, but it is useful, even if it has a few bugs and some missing fields.
Video tutorial on how to create a JSON file
Watch this video if you need help getting started with the creation of a JSON file.
Video tutorial on how to use Curbal's JSON template
Watch this video if you need help getting started with the file. I will do a tutorial on the JSON file in a later video.
Link to JSON file
And you also get my work-in-progress Power BI JSON template (It is saved as a TXT, just change the extension): Curbal_PowerBI_theme Hopefully you will find this template as useful as I have and it gets you started creating Power BI templates in no time 
/Ruth
