Lightening can certainly be done, it just requires time, effort & money of course. Refinishing of woodwork is as much an art as it is a science.
It really requires experience to get it right, and the canvas that your working with
is an expensive one. Once teak veneers get messed up or blotchy from uneven staining its hard to get em right again. So taking your time & working in sections is a must.
I would start stripping the original finish stain & varnish with a commercial stripper, or you can buy less noxious citrus based strippers, used in a less conspicuous area like the V birth cabin bulkhead, or in the head. Strippers may not get all of the stain out if its seeped deep into the pores of the wood. I would be hesitant to heavily sand the bulkheads because they are not solid hardwood. It is a thin veneer of teak and/or mahoghany glued to marine plywood. Over zealous sanging can burn through the veneer if your not careful. You must remove all the original finish in order to get a uniform new appearance. Any original varnish will not take new stain evenly & will look blotchy & terrible. This is why a lot of sailors just leave there's alone & don't fix it if it ain't broke. But when you get the off finish removed you can apply a clear varnish for a natural look, cetol for a deeper, more orange / tobacco look, Captains or Bristol Varnish comes in tints & shades, Epiphanes, etc. There are a LOT of varnish & finishes out there for teak, as everyone wants your money for buying $40 - $80. quarts of varnish.
I like to leave my teak natural without gloss in the cabin as it looks good like real wood should, & too much gloss makes pots & handrails slippery. Good luck.
