Is it possible to have italic text in Header Pro, perhaps with CSS?
It is possible yes, but first let me explain why I don't make it a one click option.
By, italicising (or bolding) a font merely by using a bold or italic tag in the html (or a css style to do it) you will be asking the browser to faux render that font.
What this means is that you have not actually got an italic variant of the font loaded and so the browser makes its best attempt at either slanting or making the existing font heavier. This always results in less than ideal rendering of the font with jagged edges to a greater or lesser degree.
The Pro stacks are all about quality typography and as such have full facilities for loading web fonts and even sub-setting Google fonts (Google Fonts are dubious when we are speaking about quality - but I'll let that go for now).
So, if you want and italic header, then clearly you should use an italic font for it in order that it will display to the same quality as the rest of your page.
With all that said if you really want to force it to faux render then simply add a custom class (the very bottom setting). For example if you entered,
italic-header as a custom class, you could then add the following CSS either to the page inspector or site-wide if required.
font-style:bold; will make it a stage heavier.
So, there you go, typography rant over You can do it - but it is a low quality short cut and you should really make a small subset of the font to handle such situations.