How to change style of a field in a specific module based on another field, v7.7

Hi,

i'll like to do the following:

 In the Contacts module i've a new custom field, a simple checkbox called leaved_company.

I'll like to change the style of the contact header in the record view (so first name last name), based on the value of this field. If checked then apply style xyz on the field. 

I suppose i've to make some custom files in custom/modules/Contacts/clients/base/views but i can't really find which ones and with what for a content ? Can i just override the part displaying the name of the contact or i've to make the complete view again ?

Thanks.

jb