the only Icons that the Theme adds is for the theme elements, eg. Hamburger menu, search, terms etc. None of which are part of the editor.
The GenerateBlocks plugin does support icons in its Buttons and Headline blocks. They support inline icons. We have plans to add an Icon Library to the Pro version at a later date.
you can add whatever SVG icons you want to a GB Headline or Button block in the SVG HTML field it provides. This doc explains extracting the HTML from an icon: