First of all you should go to this location:
wp-content / plugins / bbpress / templates / default / css / bbpress.css
Copy-/Paste the bbpress.css file into your Theme’s folder.
Like so:
wp-content / themes / [ your-theme-name ] / css / bbpress.css
When there is none css folder inside you theme folder just create one.
What you’ve done right now is created you’re own custom stylesheet for bbPress that can’t be overwritten by software updates.
What styles you have to change I can’t tell from looking at your picture, but it’s pretty easy to find them.
Make sure you have a Webkit browser installed on your computer ( Google Chrome is reallt the best for this ). View your website and right-mouse-click on the element you would like to change and pick the option ” Inspect Element ” ( I think it’s called )
Now you have at the bottom all the style , there’s also a location + line number where to find the elements inside your bbpress.css file.
Look at this video for some instructions.
It’s not that difficult.