bbPress.org

it’s coming from bbpress stylesheet.

there may be a quicker way to remove this, but one that should work is as follows.

I don’t think you have a childtheme, and now may be the time to consider having one, see https://codex.bbpress.org/step-by-step-guide-to-setting-up-a-bbpress-forum-part-2/ for more details

once done you need to navigate to

wp-content/plugins/bbpress/templates/default/css/bbpress.css

copy this file into a folder called css within your theme, so you end up with
wp-content/themes/mychildtheme/css/bbpress.css

where mychildtheme is the name of your childtheme

Then edit every line that has

background-color : xxxxxxx

change to

background: none repeat scroll 0 0 rgba(0, 0, 0, 0.4);

NOTE it goes from background-color to background
and it is only the background color that you’ll be changing, not lines which just have color: as that’ll be text!

eg

#bbpress-forums div.bbp-forum-header,
#bbpress-forums div.bbp-topic-header,
#bbpress-forums div.bbp-reply-header {
	background-color: #f4f4f4;
}

becomes

#bbpress-forums div.bbp-forum-header,
#bbpress-forums div.bbp-topic-header,
#bbpress-forums div.bbp-reply-header {
	background: none repeat scroll 0 0 rgba(0, 0, 0, 0.4);
}

This will make the background transparent, but with a darker tint shading that you have in your sidebar (the amount of difference is on the 0.4 bit, 0.1 is the lightest, 1.0 the darkest).

Come back if that doesn’t fix it, or for further help

ok, think the ones on the page link are around line 90 (my .css is different, so exact lines may vary) – look for

#bbpress-forums li.bbp-header,
#bbpress-forums li.bbp-footer {
	background: #f3f3f3;
	border-top: 1px solid #eee;
	font-weight: bold;
	padding: 8px;
	text-align: center;
}

#bbpress-forums li.bbp-header {
	background: #eaeaea;
}

and change background to the same as above.

They may be some others as you go through. Come back with any that are still niggling, and I’ll chase them down for you.

As you are making changes to bbPress templates, make a copy of the file onto your PC, and keep a list of what you changed, as any bbPress updates will override this file.

You should revisit child themes when you get some more time, as this is a better way to do it, but the way you’re doing will work fine (and was what I did in the early days!).