If you are using MineUI or even if you just want to have
all the characters needed to make boxes then you should
delete the original displaygui.png and rename the other/
remove "_MineUI" from it's name.

The reason this is needed is that the default character
map is missing a horizontally centred vertical line and
a centred cross. The provided alternate character set
replaces the characters normally found at code 5 and 7
with these missing characters.

More information about MineUI:

http://integratedredstone.wikispaces.com/MineUI