fixed some minor dark-theme errors (#602)

Author: jitendravjh

theming/theme-dark.scss

@@ -85,6 +85,9 @@ a, pre code {
     color: $links !important;
 }
 
+pre {
+    color: $foreground !important;
+}
 a:hover {
     color: $links-hover !important;
 }
@@ -119,6 +122,9 @@ p {
     color: #6c757d !important;
 }
 
+.ansi-bright-black-fg{
+    color: $foreground !important;
+}
 ::selection {
     color: $links-hover;
     background: $background-nav;