Is it beneficial to play themed practice games? While practicing, is it advisable to play themed games? For example, playing practice games using the Sicilian Defence, or the French Defence. 

I play at an intermediate level. I have chess.com rating of around 1500. I have never read or studied any chess books.
It really depends on what your end goal is. Themed games in a specific opening are good to get familiar with a specific opening, and get a "feel" for the positions arising in the resulting middlegames. So, if you want to learn a new opening, or are having trouble in a certain line, it could be a great way to learn an opening, coupled with studying some opening theory (on the 1500 level the amount of opening theory needed isn't that great, and a few lines together with some basic knowledge of ideas in the opening usually is enough).

However, if you want to become a stronger player all-round, I would say that it's an inefficient training method in and of itself, and should be used as a supplement, rather than your main means of exercise. You will learn more studying tactics and analysing your games, since tactics will be an area any 1500-level player can improve, and practicing analysis is key to being able to improve later on. But of course - play chess! That is probably the single most important means of training available, since it gives you more experience. And playing some of these games in a pre-arranged opening line will not be a waste.