Fix bad syntax highlighting color schemes (#630)

Some pygment color schemes (like `bw`, which we use as default in the
exampleSite) do not set a foreground color explicitly, which makes those
themes look bad/illegible when using the dark mode of hugo-coder. Most
such themes are light themes (it seems after a quick research), which
explains this phenomenon.

This commit adds a fix for such color schemes by setting the default
foreground color to the one used in the light hugo-coder colorscheme.
It is overridden by proper syntax highlighting schemes.

assets/scss/_base_dark.scss

@@ -38,6 +38,13 @@
     color: $fg-color-dark;
   }
 
+  // fix color schemes which do not explicitly set fg-color
+  .highlight {
+    pre {
+      color: $fg-color;
+    }
+  }
+
   pre {
     code {
       background-color: inherit;