Start by brute force cleaning.
Take a photo of the keyboard, then pop off every key cap-yes every single one. If you have access to it, clean the keycaps ultrasonically-just chuck them in a glass with some soapy water and let the ultrasonic work its magic for 30 minutes or so. After that, rinse them thoroughly(multiple water changes) and then spread the caps out on a paper towel to dry. I usually give them two or three days. If you don't have an ultrasonic cleaner, clean each keycap by hand.
While you have the keycaps off, put the whole keyboard under the faucet and use a scrub brush to clean all the crap out of the nooks and crannies. Again, give it a few days to dry.
If you're still not happy with the appearance, you can try Retrobrite-the formula is online.
Once everything is cleaned to your satisfaction, be sure it's dry and put it back together. The keycaps snap back on.
BTW, I don't know which keyboard you're talking about, but if it's the M7803 Pro Keyboard you'll want to disassemble it to access the colored bezel. You may need to Retrobrite this also.
The above will generally get the A1048 looking good.
BTW, I've been working on an IBM Model M the last few days and have folks amazed that it's even the same board.
Cleaning a keyboard well takes a while, and there are very few I will even bother doing it with. I took a Bondi Blue KB(M2452) down to the membranes a few weeks back. It had several dead keys, and I found evidence of an old orange juice spill-once the membranes were clean(deionized water and Kimwipes) and the board put back together, it worked perfectly. In general, I only bother cleaning Apple keyboards and IBM/Lexmark/Unicomp Model Ms. I also rebuilt key switches on Apple Extended Keyboards, a process which involves unsoldering the key switch from the circuit board, disassembling it, and adjusting the spring pressure to get it working again. Commodity PC keyboards are better off in the dumpster when they are dirty or don't work completely.