Does anyone know when the soundtrack album for Season 8 is released?

They've only just released the soundtrack for Day and Time of the Doctor so the smart money says a while yet. That said, they're usually quite good at fairly swift releases, Series 5, 6 and 7 were all within a few months of broadcast ending and all in time for Christmas. It's possible they'll push it back for Christmas next year but otherwise maybe April?

Does Peter Capaldi's Doctor have his own 'theme' on the season 8 soundtrack?

Nine and 10 shared 'The Doctor's Theme' and 11 had 'I Am The Doctor'.

Is there a theme for 12?

im assuming this is it https://www.youtube.com/watch?v=bF1Ng0XCUyw

Gold said Capaldi was at the recording when they were doing "Twelve's Theme" - whatever it may be called - so it seems likely to be some mix of the above link with some of this variant thrown in at a guess.

( that piece helped make certain sequences in Time Heist / Caretaker / Kill the Moon / Flatline for me )

Me too CD, I love it as a theme... The interesting thing about the piece from Flatline you linked is that the beginning is actually a '2d' version of the theme... Essentially with some 'dimensions' or tracks missing... I love how clever Murray Gold is... I know some people moan about his 'sameness' or reuse of tracks but he doesn't actually make all the final cutting decisions about which tracks go where (as evidenced by the use of Amy's theme for a conversation 11 had with Clara in the short scene at the beginning of 'The Ultimate Guide')

I agree. Murray Gold has done it again and made another fantastic piece of music.

So Peter Capaldi's Thirteenth Doctor theme will be on the next CD then...

Not official, I'm afraid - but this seems to be one of the cleanest versions of 12's theme / variants. Though I'm only listening through one half of broken and terrible earphones so I could be wrong - maybe all the muffled sound effects and dialogue are coming out the other ear

https://www.youtube.com/watch?v=e6_AzEzvHaQ

Don't expect the CD to come out yet, as we've only had The Snowmen, Day of the Doctor and The Time of the Doctor soundtrack albums out three months ago. There is nothing online about the S8 soundtrack at the moment.

Going by how Silva Screen have been releasing the CDs recently, I'd expect The S8 CD not to be out for another nine, ten months yet.

However, if anyone wants to visit the Silva Screen Records site, or the DW part of it, then this is it:

https://silvascreenmusic.greedbag.com/dept/~dr-who/