Edit or add category under Knowledge Base / Categories and insert icon name (for example "loreico loreico-heart", without quotes) into Category Icon field.
You can set global header background image under Appearance / Customize / Header Settings with Background Image option.
You can override this image for Knowledge Base pages under Appearance / Customize/ Knowledge Base with Background Image option and for FAQ pages under Appearance / Customize/ FAQ with Background Image option.
Also, if you set up a featured image for a regular page, it will be used as a header background for that specific page.
You can use free Attachments plugin for that
This theme was built with best WordPress practices in mind, which means that generally any 3rd party plugin should work. But since there are thousands and thousands of plugins in official plugin repository (and other thousands of premium plugins on various marketplaces), it is not possible to make sure it really does work with every plugin out there.
Please read the official tutorial on WordPress Codex
Importing the demo .xml file is only a first step when achieving a demo look. You have to set up your menus under Appearance / Menus, edit widgets and sidebars under Appearance / Widgets and set up your front page and blog page.
Probably the most important step is to set the demo front page (page with title Knowledge Base. Elegant. Lightweight. WordPress Based.) as your site's front page. It can be done under Settings / Reading. Then you can fine tune it under Appearance / Customize / Default Front Page Settings.
There is a separate article about the front page. You should definitely read it.
The easiest way to do it is to install one of many 3rd party plugins for that.
Yes, theme was tested with bbPress plugin. You can read more about its integration in Knowledge Base.