I thought that setting a <Text> to bold would have been simple as adding:
Weight="Bold" Style="Italic"
but, as it did not worked, after looking to the Docs and to the examples I understood it isn’t.
As I prefer to use the system fonts (Android and iOS), I found that there’s the <Systemfont> class that may be used. Taking the following code from the Docs I thought that I could change the Weight to Bold, but again this appears not to be the correct way:
Seems to me that you’re doing the right thing, only that italic and bold are not available for the monospace font. (At least on Android, where I tested).
If you switch from monospace to sans-serif then it seems to work as expected.
A new way of setting bold/italic for system fonts has been implemented and rolling out soon; it will basically be <Text Font="Bold", <Text Font="ThinItalic" etc
Beyond the system font, things get a bit tricky in a cross-platform perspective. There is no guarantee that any other fonts are available cross platform, so you have to provide your own .ttf of the fonts (and weights/styles) you want to use.