wlt: theme: change frame color to white plus black border

It is currently pretty annoying to use multiple wlterm windows stacked on
top on each other when the terminal-background is black. The border
doesn't use multiple colors so it is hard to distinguish from the
main-frame.
This patch changes the border color to white (as most terminal-backgrounds
are black by default) and additionally draws a black 1px frame around it.
This guarantees that the frame is even visible with white terminal
backgrounds.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>