Hello beganovich,
I am using an older version of Linux Lite so do not have access to the stylesheet.
You can try to add this code (see final reply in link), it is for the bottom border but it can be adapted for sides and top -
https://stackoverflow.com/questions/4114552/how-to-remove-the-bottom-border-of-a-box-with-css/4114596
Also changing the border colour to the same as the background of box should make them disappear in another kinda way
I am using an older version of Linux Lite so do not have access to the stylesheet.
You can try to add this code (see final reply in link), it is for the bottom border but it can be adapted for sides and top -
https://stackoverflow.com/questions/4114552/how-to-remove-the-bottom-border-of-a-box-with-css/4114596
Also changing the border colour to the same as the background of box should make them disappear in another kinda way