I have a red border around my modeline, and I don't like it. How can I get rid of it?
(set-face-attribute 'mode-line nil :box '(:width 0))
Like upper code shows, you can change
mode-line face, or
mode-line-inactive. set border
0 or change border color to the same color with mode-line background.
You'd better take a screenshot to show how it looks like.
Options > Customize Emacs > Specific Face >
mode-line > Box around text