If I may rephrase...
How can you Modify properties of the Style set as the current style?
you would use it's objectID to open it as a TextStyleTableRecord and modify it.
How can you Set the Current text Style to a different (existing) text style?
The Database.Textstyle property is not listed as read only, so you should be able to assign it the ObjectID of the Textstyle you want.
to get the ObjectID, use the Item property of the TextStyleTable.
Dave O.