So it is in there, but possibly not obvious as it doesn’t look like a colour within that code.
It is this line of code:
border-left: 1px solid var(--background-modifier-border);
The actual colour in that example comes from a variable (var(--background-modifier-border)), which is set elsewhere. However, you can change it in the CSS if you find it within your theme.
Changing it like this would create a bright-red line:
border-left: 1px solid #ff0000;
Changing it like this would make a green line:
border-left: 1px solid #00ff00;
etc