So - first task is to copy the template /templates/desktop/spForumViewDesktop./php from the Reboot theme folder into your child theme in the same location.
Now you need to make an edit to that template in your child theme. Look for the comment -
# Column 3 of the forum row
at line (about) 198. Nite this is the second time this comment is made. The first is to do with subforums.
In the correct block of code, aftyer the column start you will find the function
Track down 2 or 3 lines until you find the first call to sp_InsertBreak(). Immediately AFTER that line of code add the following new line:
This needs to be entered EXACTLY as is shown here. But you can adjust the size (set to 25 pixels here) to whatever you prefer and what fits best.
You may want to do something with the mobile template as well. But of course, the layout is different.