improve inline code contrast across all slides

- change code text color from #1e5f8a to #5fb3e4 (brighter blue) in course 223015b
- update code styling in course 223015c for better contrast
- update custom-theme.css with --color-dimmed variable

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

themes/custom-theme.css

@@ -6,6 +6,7 @@ section {
   background-color: #f0f8ff;
   color: #2c3e50;
   font-size: 0.85em;
+  --color-dimmed: #e0e6eb;
 }
 
 h1 {
@@ -21,13 +22,17 @@ ul, ol {
   font-size: 0.8em;
 }
 
-code {
-  background-color: #ecf0f1;
-  color: #2c3e50;
+section code {
+  color: #1a1a1a;
   padding: 2px 4px;
   border-radius: 3px;
 }
 
+/* Preserve explicit SVG fill colors */
+section svg *[fill] {
+  fill: revert;
+}
+
 blockquote {
   border-left: 4px solid #3498db;
   background-color: #ecf0f1;