A lockable corner bracket includes a lockable encapsulated axle pulley which houses one end of a length of cord, with the free end secured to a side bracket. A cord locking mechanism is used to lock the encapsulated axle pulley and prevent unwinding of the cord, or loss of tension, during transport and installation to facilitate do-it-yourself installation. The axle pulley is further configured with a central slot to allow frontal insertion of a roller tube. The corner bracket may be used with a manual roller screen including roller tubes fitted with spring assist systems to facilitate winding and unwinding of the screen, or with roller tubes fitted with a tubular motor to drive rotation of the roller tube.