Conditional formatting applies a format to the entire layout object, whether that be a field or a block of layout text (that what you mean by "unassigned text"?) Therefore you can't use a conditional format to highlight selected text within the object.
You may be able to use Find/Replace in a script to "find" and then Highlight selected text in a text field, but this won't work for Layout text objects and it will only find the first such instance and highlight it, not all of them.
A calculation field using substitute can be used to subsitute formatted text in your field for matching unformatted text. This can mark selected text with a different color or make it bold face or italics, but substitute is case sensitive--so that complicates the approach as well.
A calculation field that returns text like the following may work:
Subsitute ( TextField ; [Upper ( Matchtext ) ; Upper ( TextStyleAdd ( MatchText ; Bold ) ) ] ;
[ Proper ( Matchtext ) ; Proper ( TextStyleAdd ( MatchText ; Bold ) ) ] ;
[ Lower ( MatchText ) ; Lower ( TextStyleAdd ( MatchText ; Bold ) ) ] ;
[matchText ; TextStyleAdd ( MatchText ; Bold ) ) ] )
This isn't perfect as "ADb" won't cause "adb" to be shown in bold face though most conventional capitalizations will be shown in bold face.