@dboing What you are asking is indeed too much to ask or expect from a simple forum reply.
If you wish to understand the depths of it you will need to study because what you’re asking is somewhat similar to someone asking to be taught endgame technique within a relatively short post and that’s just not a realistic thing to ask or expect.
However, I will contribute a bit:
Themes are moves, maneuvers or combos that occur again and again within the various different variations that are stemming from a particular setup. If, for instance, you see that in your analysis a knight is almost always best placed in a specific square (for positional reasons) it becomes a theme of said setup.
However, a “thematic opening” is also one where it is suggested that straightforward (and classical) development, at the very least, works. Or at least, one where the things that work work in it.
But… It also suggests that it is affected by the pawn-structure and so thematic guidelines are often themes that are associated with said pawn-structure.
The book Mastering The Spanish is a book about the Ruy Lopez’s pawn structure which in return calls for development themes.
Within the world of pawn-structure you will find a world of themes so you can always draw from previously studied positions and apply what you know to the position at hand (if you listen to Carlsen’s post game analysis he almost always gives such examples). This, in essence, is what a ‘theoretical position’ might be. That is to say, a position that is extremely potent, has themes (usually derived from the pawn structure) but can often be stirred in different directions each of which introduces its own complexities.
A grand example to a massively theoretical thematic position is the Semi-Slav (Botvinnik variation): 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Nf3 c6 5. Bg5 dxc4 6. e4 b5 7. e5 h6 8. Bh4 g5 9. Nxg5 hxg5 10. Bxg5.
There’s so much analysis that went through here that entire world championship matches were decided on who understood it better.
So, to ask for a detailed explanation is something that in all honesty I’m not sure I know how to give in less than six months of talking whilst providing dozens of variations.
What I can say is that after: 1. d4 Nf6 2. Bg5 g6 3. Bxf6 exf6, in my opinion, black is completely fine as long as he can play ...f5 and so far I haven’t seen anything stopping black from doing exactly that.
The remark made by
@HDGhog that black’s position is “passive” is somewhat misplaced in my personal opinion because to start with the Trompowsky is passive to begin with so it will obviously be followed by passivity in comparison to say… the KG.
Also, trying to force sharpness on a position, while it is a topic for a different debate, is often double edged or dubious.
We can debate on whether it’s playable after 1. d4 Nf6 2. Bg5 or not but, and this is a personal side note, I’ve always thought these comments are too individualistic. A bit like how some people are attracted to the French or Caro Kann and others can’t imagine ever playing them.
To me, these things are so specific you can barely even talk about.
I actually like closed positions and I do well in solid structures I can work with slowly building up the position as a whole.
I know a player who loves the following variation in the CK: 1. e4 c6 2. d4 d5 3. Nc3 dxe4 4. Nxe4 Nf6 5. Nxf6+ exf6.
Most people will never adopt this as their main defence but the player in mention analysed it so deeply they feel comfortable in it and that player once told me that playing boring positions makes people so crazy they start making mistakes or initiate an untimely attack which results in them losing games.
On the other hand, I myself stopped playing 1.e4 altogether due to the Berlin because I find it “passive” and “boring” to a level that I would rather not play chess at all.
Going back to our discussion, after: 1. d4 Nf6 2. Bg5 g6 3. Bxf6 exf6, considering ...f5 can’t be stopped, themes of such structure are pretty straightforward:
...Nd7-f6-e4, depending on whether you (as black) play ...d6 or ...d5 you may want to place the LSB either on e6 or b7, you actually don’t always want to play “religiously” by playing the DSB to g7 because it can sometimes find a better home at d6 supporting a future ...f5-f4, Rooks on e8 and d8, Queen will usually be kept at the d8-h4 diagonal (seeing as ….f5-f4 is a thematic leaver/attack). You can take a lot of themes from the Catalan and use them as black or even some lines as white in g3 lines of the Slav.
Is it slower and “more passive” than other lines in other openings… sure… but how is it bad or losing for black?
If you could just break black’s position so easily the Catalan wouldn’t exist as well.
I actually think it is easier to defend against h4-h5 attacks in this position than it is in the Gruenfeld.
The point I was making from the start (and it is perhaps the only point I was making) isn’t that 2...g6 is great or best or that everyone should play it because it should be everyone’s favourtie.
That’s not what I was saying at all. It’s not even my favourite!
What I was saying is that calling it “bad” is simply wrong because at its essence what renders a position favourable or equal is the ability to be rid (or use) of “weaknesses”.
If you can’t get rid, use, or trade off your weaknesses then your position is worse (or even in grave problem). If you can then you should be equal.
In this case the very question is whether black can play ...f5. If he can and if it can’t be stopped, then black can equalise and if that is achievable then calling it bad because of personal preferences is wrong for analytical reasons.
I can easily make arguments for why 2...e6 followed by ...c5 is far more miserable and that too while being accurate will still be a personal preference because I would only make an argument to why I personally prefer one kind of weakness against another kind of “miserable” weakness.
For instance, I think both the Modern setup as well as the Penrose variation have completely refuted the Benoni, yet, people still play it.
