CAN you install a standard shower light? Certainly. Will it last? There are no guarantees. It could last 3 weeks or it could last forever. Most standard fixtures have not been tested in this environment and will not be warrantied for this application. If the fixture fails, you will have to replace it at your own cost.
You can install a fixture constructed of solid metal with a glass lens and seal it with silicone, but you will need to break the seal and then reseal each time you have to change the bulb. Going back and installing a different fixture down the road can be pricey and frustrating.
Whichever way you decide to go, you will be delighted with the ambiance lighting adds to a steam shower!
Read more about steam shower lighting here www.steamsaunabath.com/catalog/all_about_steam_rooms_lights.php#lights
Find shower lights approved for installation in steam showers here www.steamsaunabath.com//catalog/index.php/cPath/64_80