Depending upon what's on there now and how much darker you want it to be, you might be able to get by with a glaze.
You can use a glaze to darken (within limits), alter the hue, and "dirty up" the finish to give it a richer look. Glazing is very forgiving because you can wipe or brush it to get it your satisfaction, or even remove most of it during the process.

You have to cover the glaze with another coat of finish, one compatible with both the base layer and the glaze.

If this was already your house, you could experiment with glazing the inside of a door somewhere. But since you have not bought it yet, you might have to look at the photos below and take an educated guess.

The other technique is to use a toner, a finish with color in it. However, you almost always have to spray on a toner to get an even color. You need to apply it light and with a good deal of skill (i.e., experience) Do not be tempted to use the all-in-one products such as Polyshades. It's nasty stuff and very difficult to get to look good at all.

Here is a link that might be useful: glazing

The first thing you need to do is determine the type of finish that is already on the wood. Then you need to meticulously clean it and find out the finish is in good shape and adhers well. Repair any damage such as chips, scratches, marks, water damage, ect. After all is canny, you are ready to stain to give it a light sanding and stain.
If the finish on the wood is oil based, you can use a lacquer stain or alcohol based stain. These must be sprayed on. They way you save money is to do the prep work and taping off yourself. Then the stain needs to be applied very lightly so it doesn't raise the original finish.

Rosewest
Not a stupid question at all and I am sure the responses you received will help a lot of folks including me. johnny