Not sure which part of the term you are talking about - you mean using Lua, or modifying CC directly, to change the yellow/gray border around the term?
In Lua, that's simple:
width = 2
term.setBackgroundColor(colors.red)
w, h = term.getSize()
for x = 1, width do term.setCursorPos(1, x) term.clearLine() end
for x = h - width + 1, h do term.setCursorPos(1, x) term.clearLine() end
for i = 1, width do
for x = 1, h do term.setCursorPos(i, x) term.write(" ") end
for x = 1, h do term.setCursorPos(w - i + 1, x) term.write(" ") end
end
If you mean the actual gray/yellow border, you can unzip CC in the mods folder, and go to mods/ComputerCraft/textures, and in each of those folders in there are the image files for the computer, the borders, etc… Under GUI is the borders around the term.