Hello Hugo,
For the selected item, you can change the background colour, give it a border or change the font colour, but you can't change the font to bold or italic.
To change the selected text font colour (which is about the nearest to what you want to do), go into Tools-Options-Themes and Formatting. After clicking the Edit button to bring up the edit window, select the "Tree format" tab and the "Selection" tab under that. All your formatting (colour) options are on that tab.
Stéphane