Im pretty new to coding, so what are the limitations of using HTML? Additionally, I have tried using accent color like <title source="Title"; accent-color-source="#000000">, but it does not change the color of the background. Sorry for my naivity.
Portable Infoboxes use a special templating syntax, which is not HTML. It's not possible to mix and match attributes. The accent-color-source can only be used inside the <infobox> tag, because it's an instruction for the whole Infobox, not only a single component.