Add new style in WYSIWIG Editor

I need to define a couple of new text styles in WordPress. As an example, let’s take introduction, a style that could be used for the short introduction text below a headline. As standard I would want to define that as maybe “paragraph text + italic”, but sometime down the line, I might need to redefine that as “paragraph text + bold” (or whatever). In other words, it should work exactly like H1.

I need the new text styles to be available for selection from the WYSIWYG page/post editor, again, just like H1.

I have trawled this forum and Google in general for hours now, and I can’t come up with a solid answer. Some answers seem to only work in old versions of WP, others require a lot of hacking/coding in several config files.

I cannot believe that there is no easier way; there should be a plugin for this somewhere if nothing else, but I cannot find it. I have been pointed to TinyMCE Advanced, but cant get it to work in that way (even though it’s useful in many other)