There are two status.
status 1: wp-syntax is active.
vim /wp-content/plugins/wp-syntax/css/wp-syntax.css
.wp_syntax pre {
border: 5px solid red;
}

status 2: wp-syntax is inactive.
vim /wp-content/themes/twentysixteen/style.css
code,
kbd,
tt,
var,
samp,
pre {
font-family: Inconsolata, monospace;
}
pre {
border: 5px solid red;
}

How to make pre border format to be :5px solid red such as status 1?