Got this 1040 STe (4mb upgrade) a couple of years ago, very yellowed / discoloured.
Stripped down and removed all keys, motherboard etc. Keyboard in pieces. Then outside on the table and painted case and keys with bBlonde peroxide hair gel
Continually through the next few hours, applied recoats and moved the table (base was on a chair) to follow the sun and rotated 180 degrees every 30 minutes or so to ensure all sides were equally sun soaked. Wash off in the bath of the pieces and then back out for another hour. Keys not completed (still yellow in places) so will need another go next sunny day. Case and under keys element done.