Improve .md code color (#1918)

## Description

Inherit from top-level color schemes so that the text color is readable in both dark and light modes

## Does this PR need a docs update or release note?

- [x] ⛔ No 

## Type of change

- [x] 🐛 Bugfix
- [x] 🗺️ Documentation

website/src/css/custom.scss

@@ -195,7 +195,7 @@ html[data-theme='dark'] {
         display: inline-block;
         line-height: 1.2em;
         vertical-align: top;
-        color: #e74c3c;
+        color: var(--ifm-color-primary-dark);
         background-color: unset;
       }