Remove unused template and old theme files

Deletes files no longer referenced after modernization:
- right.html sidebar partial (removed during layout cleanup)
- Old JBoss theme CSS files (jbossdeveloper-thin.css, styles.css, wide.css)
- Old theme background images (background.png, bg.png)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Signed-off-by: James R. Perkins <jperkins@ibm.com>

Author: jamezp