Jill Cordner, an interior designer in San Francisco, says you can mix styles, especially if the new elements are clean-lined. She suggests the following:
We are proud to be your Centennial electrician, and by being in this area, we see different aged houses all of the time. What if you like modern lighting but live in an older house? Below are some suggestions on designing lighting in houses like these.
Coordinate finishes:  If you have a silvery mirror or sconces, stick with that finish. Darker and warmer metal finishes mix more easily than, say, chrome and antique brass. Black goes with almost anything.
Go for a simple shape:  A clear globe or linen drum is foolproof. Or replace a candlestick chandelier with a Sputnik-style fixture—its touch of glamour translates well to a traditional interior.
Pay attention to scale:  Add up the length and width of the room in feet; that number, in inches, will give you the ideal diameter for the fixture. Or, if the fixture will be over a table, make it a third of the table’s length.
Mixing Styles in a Traditional House | This Old House