You need to edit a file called
themerc. This file will be in
/usr/share/themes/ThemeName/openbox-3 or in
~/.themes/ThemeName/openbox-3 where ThemeName is the name of the theme you've chosen. If you haven't switched themes and are using a default Lubuntu as you've stated, your themerc would be
The first few lines of the themerc hold the answer:
Default Lubuntu theme for Openbox, GPL2 / Open Source
By Rafael Laguna, Lubuntu Artwork Team
menu.items.active.bg: Solid Flat
After making a copy of the original themerc for safety, you'll need sudo to edit the file to increase
1 to whatever you're comfortable with. Run
openbox --reconfigure to make the change effective (after you've edited and saved themerc.) Any window you open after this will have the new, thicker, border width. No need to log out, reboot, etc.
If you are happy using the mouse, you can resize a window from within its corners somewhat easily.
If you move your mouse cursor from within the window towards any corner, you'll notice it changes appearance to a right-angle with an arrow inside.
At this time, you can press the left mouse button and drag the modified cursor to resize the window anyway you like.
This works even when the window borders are hard to grab from the sides or top or bottom.