You'll have to use the Excel,Application COMObject or the ImportExcell PowerShell module to do that.
CSV file are text-only and contain no "formatting" at all.
You'll find plenty of examples on manipulating cell formatting using the Excel.Application by searching the for things like "powershell excel com object", "Powershell Excel Automation", etc. The problem with using that is that it can only be used on a machine that has Excel installed on it (which eliminates servers).
The ImportExcel module doesn't depend on the presence of Excel and is, to most people's way of thinking, easier to use.