In an "on-premise" environment you can update a file called custom.css on the application server hosting business intelligence then you reference the classes you have added in that box on the column properties "CSS Class". So you can create a class then use on on many reports. If you then change the class it gets updated on all the reports. I assume this is not possible on the cloud! However to can hard code you style in the other box "CSS STyle". For example,  I use white-space:nowrap; to stop text in columns wrapping if i know it will never get too wide such as on a status column or a date column.