Tweak code CSS a bit to make it nicer

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

assets/scss/modes.scss

@@ -15,6 +15,7 @@
         --target-background-color: #{$target-background-color-dark};
 
         --code-color: #{$code-color-dark};
+        --code-lineno-color: white;
         --code-keyword-color: #{$code-token-color-dark};
         --code-type-color: #{$code-token-color-dark};
         --code-highlight-color: #{$code-highlight-color-dark};