There is an adjustment. There are two small knobs at the bottom edge of the instrument cluster. However, unless you're lucky this is probably not your problem. The instrument lights, even if perfect, are not very bright.
The instrument cluster is very easy to pull out, There are no fasteners, it just pushes or pulls out. Be careful prying on the edges, you can break the plastic lens loose. There are three screw covers at the front edge of the under dash cover. Remove them and the three phillips screws under these covers, there are then two plastic fasteners with phillips slots in them at the bottom of the underdash cover. Turn these 90 degrees either way, then pull them out.
After removing those fasteners, lower the under dash cover and reach up behind the instrument cluster and push it out. If you want to take it all the way out, which may not be necessary, disconnect the speedo cable, oil pressure line (10MM open end or tubing wrench) and unplug the round electrical connector. There will be, I believe, one more lamp on the right side, that you will pull out. Then you can take the entire cluster to the bench and replace all the bulbs. Some of the bulbs are in small black plastic carriers that twist in.
I just described the time consuming way. You can if careful, and part orangutan, replace the bulbs without entirely removing the cluster.
I don't remember how many bulbs are present, but I think about five.
Good luck with it,
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in