you would have to edit and copy files from one to the other.
since its bbpress its mostly going to require copying the template files and bbpress.css , but i am not sure if your going to going to need a function for bbpress in the function.php in the theme or many more codes.
This could be really risky if you dont know what you are doing.
I recommend working on a local development area. WAMP for Windows , MAMP for Mac , LAMP for linux , and XAMP for all.
Help on this is really minimal on my side. Its more something you have to do yourself.
But if you do need help transferring i might come up with something.
Other than that it might be an answer of i dont have the time to this kind of thing right now.
I kinda undestand what you mean but now that I read again, I’m lost. Can you give me some steo by step instruction ?
The information i gave you is if you wanted to change your theme to another but still keep the way bbpress is styled.
IF you wanted to just be able to change themes while bbpress stays the same throughout all of them this is not possible right now.
where are you lost ?? i basically said its copying the bbpress files from one theme to another.
i mentioned that you might need to copy some functions from function.php.
Stargazer Colloquium child theme uses get_theme_mod (which is added to the customizer area to your theme) to add certain color to your bbpress theme.
You can either delete that function and just added the colors manually to your bbpress.css or hook it up to your new theme which might be a tad more difficult.
the local development area that i mentioned more can be mentioned is great if you want to test things out like transferring files into another theme.
here if you use Windows Pc
http://www.wpbeginner.com/wp-tutorials/how-to-install-wordpress-on-your-windows-computer-using-wamp/
and here if you use Apple Mac
http://www.wpbeginner.com/wp-tutorials/how-to-install-wordpress-locally-on-mac-using-mamp/
steps umm i guess this.
1. set up local development area
2. download the files you have edited from your current theme on your website to your computer , ftp like filezilla is recommended.
3. transfer those files to your local development area , into the other theme you want to use.
4. start seeing if there is any issues. (the most likely is the get_theme_mod areas that add color) but that’s just color and you can add it to your bbpress.css.