Remove outer border of syntax highlighting

[L0g4n]

How can i remove the outer border when using syntax highlighting ?

Pic:

[ghost]

Same issue here. I would like to remove the box of the highlights.

[xealgo]

I've been trying to do the same. I modified the style in chrome dev tools, but now I can't figure out how to add a custom css file without wiping out the main theme styles :/