The CSS file for Tags Cloud is located at: DNNRoot/DesktopModules/LiveContentCloud/CloudCSS/Default.css

#LiveContentCloud
This is the parent div id for Live Content Tags Cloud

#LiveContentCloud A.LCCloud1
This is the class for Weight1

#LiveContentCloud A.LCCloud2
This is the class for Weight2

and so on...

Note that #LiveContentCloud A.LCCloud1 is defined under Administration/Configure section of the module (Bold Part is the CSS Class defined)

Examine the CSS file for all CSS Classes...