Changing Colour outline boxes

Ive never used outline boxes before can someone please tell me where i change the colour of the outline boxes on the home page when displayed in  outline.

Thank you

modules/boonex/wall/templates/base/css/default.css  -  search for .bx-def-shadow in the css file. 

thank you very much

Is there any other way to change the outline boxes? I'm afraid doing it this way it will be overwritten when I upgrade to the next version. Any solutions or I'm not understanding it right?   

I'm looking for an "elegant" solution it that makes sense.

Thank you so much.