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>

courses/223015b/slides/2025-12-19-termin-0-intro.md

@@ -31,7 +31,7 @@ h2 {
 }
 pre {
   background: #0f0f23;
-  color: #1e5f8a;
+  color: #5fb3e4;
   border-radius: 8px;
   border-left: 3px solid #1e5f8a;
 }
@@ -41,7 +41,7 @@ pre code {
 }
 code {
   background: #1a1a2e;
-  color: #1e5f8a;
+  color: #5fb3e4;
   padding: 0.15em 0.4em;
   border-radius: 4px;
 }
@@ -60,7 +60,7 @@ section.klausur footer {
 <!-- _header: '' -->
 <!-- _backgroundColor: #000 -->
 
-![bg fit opacity:0.4](./assets/digital-landscape.png)
+![bg cover opacity:0.2](./assets/radek-grzybowski-eBRTYyjwpRY-unsplash.jpg)
 
 # Dateiformate, Schnittstellen, Speichermedien & Distributionswege