No, dude.
Can I customize tactics trainer?

Change Training Mode to Custom Training (Unrated) on the website. You can select themes and rating ranges:

No, dude.
If you don't know, why did you answer?
I've mentioned the custom mode in several comments about learning tactics.
Just click on the Training Mode icon to the left of the Tactics Trainer Start button and then choose "Custom (Unrated)" and then you get the window in Martin Stahl's comment above.

If you don't know, why did you answer?
I've mentioned the custom mode in several comments about learning tactics.
I've never seen that customization, so I was expecting it doesn't exists.
So I had a wrong answer, happens!
There's been another person not knowing that customization. So I ask you, you
complain about me giving a wrong answer, the other person not? Seriously?
"I've mentioned the custom mode in several comments about learning tactics."
If I would point you on one of my posts after you did a wrong answer, what would you say.

Change Training Mode to Custom Training (Unrated) on the website. You can select themes and rating ranges:

If you don't know, why did you answer?
Right now I've checked the example given above and it seems my answer was right, for a non-pay account. For a pay-account the answer is yes, it's customizable as shown by Martin_Stahl but this doesn't apply to non-pay accounts, at least on my account.
Maybe the next time you complain about one answer better check if You are right, dude!

If you don't know, why did you answer?
Right now I've checked the example given above and it seems my answer was right, for a non-pay account. For a pay-account the answer is yes, it's customizable as shown by Martin_Stahl but this doesn't apply to non-pay accounts, at least on my account.
Maybe the next time you complain about one answer better check if You are right, dude!
Are you on the default, v3 site? That is not available on v2.

CookedQueen: when you go on the Tactics Trainer, is there a little icon to the left of the "Start" button? If so, does it show Custom Unrated? That's what I see.
In any case, the member who asked is a paying member.

problem here is the OP is a paid account while your answer is an absolute "No". I think if you had added the fact that your observation is based on a non-pay account then things will be much clear.
...
I have not observed that I cannot do the customization when I was a free loader in this site. Maybe next year I can attest to your observation... or ask a friend who can observe it for me. ^_^
No, that's not the way it's working. I was the one putting the needed distinction on the table due to the fact that according to the answer of Martin there is a good way for the required customization but obviously missing in my configurations. You can't expect that someone knows about possible settings from other kind of accounts. Some know but most are non-paid and from a perspective of a non-paid the answer is simply No, no customization. If then the answer needs to be distinguished depending on the kind of user-account, fine. I've put it on the table and still people like Mickin don't get the point that not everyone can be aware of the possible distinction.
Also I don't know your observations as a free-'loader' but on my setting the information from Martin is not fitting.

CookedQueen: when you go on the Tactics Trainer, is there a little icon to the left of the "Start" button? If so, does it show Custom Unrated? That's what I see.
In any case, the member who asked is a paying member.
Obviously you didn't know about the needed distinction to answer the question but still complaining about another answer. Instead of saying 'fine, it depends' no, you pick one single answer and refer also to your threads you wrote about tactics. Not everyone reads this, independent how helpful your tactic-posts were. Read #8 again. Your complain is ridiculous.
I have lost too many games because I get pinned and just miss it. I would like to just practice pins on tactics trainer. Is there a way that I can do this?