lol maybe u can try these (off da top of my head)
1. At the top where it says File and stuff, find Select
2. Click on it and select "Select All" (use the bg of sig, not foreground characters etc.)
3. Go to Select -> Modify Border
4. Get the px u want (preferably 1) and go to Edit -> Fill
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If that's too complicated 4 ya, try this:
1. Get pencil tool at 1 px
2. Hold Shift and drag the pencil corner to corner (Shift straightens the line so you don't have to redo it when u get ur line crooked)
3. Repeat step #2 for another side
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Both achieve pretty much the same effect overall.
NOTE: The higher the opacity, the bolder it looks. The lower the opacity, the thinner it looks. Play around with it to get wat u want.
-Kung Fu Master /Out