In a modern office, shelves barely exist; modern buildings have special rooms called archives, and folders are put away from the desk. Basically, these rooms are empty, not counting the desk and the chairs. Now, if your space is larger, like Harvey Specter's office from the legendary cit com “Suits” , than you'd like to add a few shelves not for the work folders, but for various stuff like achievements, rare records, some limited edition books and similar luxurious stuff. As for the classic office style shelving is important. And it has to be in harmony with the rest of the elements so that it creates that traditional, rich, royal look. Again, these shelves won't be filled with work folders, but mainly with specialist books. Shelves can also be ordered online, and there are companies that offer the service along with a team to assemble it. You should research before you order it and look for the best contractor.
In my opinion, these are the three main elements you need to choose wisely to design a stylish office. Both styles, modern and classic are styles, and we can't judge tastes. Younger workers will always choose the modern style offices, while people with a lot more experience and years of work behind them would like their office to express their success.