joplin is highly customizable with css. you can check out some examples here: Share your CSS. the last post has note and warning boxes in the rendered view which might be useful to you.
you can quickly access the css files by going to settings > appearance > show advance settings. then you can paste the code there. or if you want to make a small change, you can toggle development tools under help, to look for the names of elements etc. (you need to restart the app to apply changes, more info at https://joplinapp.org/ under custom css section).