Your request is not possible with a plain string data type. LabVIEW wouldn't ever pollute the concept of a string with our own encoding of fonts, styles, etc. There are thousands of encodings for such information out in the world -- some would embed the encoding into the string itself, others carry the encoding information in a cluster alongside the string. You need some more complex data structure to include encoding in the string and then you need a display function capable of interpreting the encoding.
The One and Two button dialogs are system dialogs supported by the various operating systems. Those system dialogs do not support any font information to the best of my knowledge. The Three Button Dialog.vi does support such encoding for the <b> and </b> tags, and you can adjust the VI to know about other encodings if you wished to do so.
