The Info Boxes are exactly the same that the marketing blocks. There are no restrictions in the code except the same moodle restrictions can apply.
The reason of most of the sites using Adaptable is the marketing blocks and info boxes. So they are working well and you can enter any content even forms or Google Maps.
I suspect you have some wrong moodle setting or the code is not correct.
<div style="text-align:center; background: #fff000; height: 240px; padding: 7px;">
<h3 style="color: #000000;">Title</h3>
<div style="color: #000000; text-align:center;" >Add your text here.</div>
Open Atto, copy & paste the code in the code editor (not the WYSIWYG) and save. The infobox or the block should display a yellow background with black text.
If it is not working, then open the editor, change to code view, take a screenshot and post it here.