How can I install exterior trim around my windows
The problem that you will have is that you have stucco walls, this complicates the attachment of any decorative trim (but does not make it impossible).

For your application I would suggest lightweight foam architectural trim, painted as you wish. This is commonly used around windows on homes with stucco walls (albeit it is usually installed on the "brown coat" then stucco'ed over). The advantages are that it can be attached using special adhesive, you then caulk around it and paint; any other type of trim (wood, shutters, etc.) would require drilling into the stucco and using anchors to fasten.