bbPress.org

That’s IE for you !

For some reason that I can’t immediately see, IE is ignoring the background setting, so is just saying it has no background so showing white

I’d suggest you try changing line 999 on dark.css from

/*media all*/
media screen #bbpress-forums fieldset.bbp-form p, #bbpress-forums fieldset.bbp-form textarea, #bbpress-forums fieldset.bbp-form select, #bbpress-forums fieldset.bbp-form input {
    background: rgba(18, 21, 24, 1);
    color: #fff !important;
    border: 0px !important;
}

to

/*media all*/
media screen #bbpress-forums fieldset.bbp-form p, #bbpress-forums fieldset.bbp-form textarea, #bbpress-forums fieldset.bbp-form select, #bbpress-forums fieldset.bbp-form input {
    background: rgba(18, 21, 24, 1) !important;
    color: #fff !important;
    border: 0px !important;
}

It may or may not work !