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.
For somebody at your level I'd recommend to focus on tactics and general principles (in openings/middle game and endgames) first, before worrying about specific openings (which have very little effect compared to missing simple tactics etc). Also if you learn typical general motives/principles of openings (attacking a pawn chain, developing pieces....) first they will help a lot when studying a particular opening later.
If you do want to learn an opening, I would recommend to first find out what kind of player you are, particularly whether you prefer closed positional games or rather open tactical battles. This will influence the kind of opening you are happy with.
In a second step, I'd take a look at some master games played in different openings and see what kind of positions result from different openings and which of them you like best. Looking around moves 10-15 is usually a good indication for what you get out of the openings.
If you have decided on an opening I'd start with learning the main line to some reasonable depth. Make sure not only to memorize the moves by heart but also to understand why they are made and what the general idea behind this particular opening is. For this you probably need some external help such as books, a database and online resources (videos, streams where good players comment on their moves live,...).
Once you have a rough idea of how this opening works you can think of playing themed games with an opponent. This will be a lot more efficient if your opponent has a much higher level than you or at least knows this opening much better than you; as in that case he will be able to point out mistakes you do when practicing. BTW, on lichess there are also regularly themed tournaments where you start from a custom position (randomly playing black and white): https://en.lichess.org/tournament
In any case I'd see these themed games only as a supplement and a way to test yourself. If you want to be serious about chess, it will not replace studying with databases, books, annotated games, analyzing your games for mistakes etc. Another good way to study particularly if you reach a level of Elo 2000 or so, is to pick a GM who is playing your opening and to follow their games.
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.
A great practice routine if you have a willing partner is to take a position from an opening you wish to study and play a series of blitz games from both sides. For example, when I was younger me and my chess buds would play a bunch of blitz games from B63, Sicilian Richter-Rauzer position:
We did this with the Sicilian Dragon variation as well.
This is a tremendously beneficial training technique. You learn what types of middlegame plans work (and don't work) from both sides of the board. Pick any opening position , say 6-10 moves in, and have at it.
After you have played a bunch of blitz, move along to longer time controls (G10, G15, G30 even), and help refine your play.
I think it can be very good especially if you are getting into a rut playing the same sort of game over and over again. I know that when I played too much blitz I just threw openings all over the place, but I wasn't focusing on one and learning. It became more about short term trying to fool my opponent than saying, I want to get better at this or that.
So if you see yourself or a friend flagging in this way, a training game seems like a good way to get around that. Keep notation and see what computers/books say and it'd be a very good learning experience.
