To Reproduce:

1. Install Confluence 5.3.4
2. Insert the Code Block Macro onto a page with midnight theme
3. Insert a second Code Block Macro with midnight theme
4. Save the page
5. Change the theme to Confluence on the top Code Block Macro
6. Save the page
7. View the page and see that both Code Blocks display the midnight theme

Materials attached:
Screenshots of the Code Block Macros in the editor and as displayed