Option to change the text or the style of part of the text

Now we have only two option to change the style: one is to change all text style which is like control-a and another one is to change the text from now. I want to have an option to highlight part of the text and thus change only it

Start by tracing the existing controls for changing all text styling and styling text from the current position. Determine how highlighted text is represented, then make the requested behavior complete when styling affects only the selected portion rather than the entire text or text after the cursor.

Written by the indexing model from the issue text.