Those are images inside the theme package. To access and edit them, you'll have to unzip the theme. Check this topic in Theme forum:
Required knowledge : Unpacking a theme
https://forum.cockos.com/showthread.php?t=172341
Please note that mcp_master_fxlist_empty and mcp_master_sendlist_empty images are not included in every theme. In those cases theme uses mcp_fxlist_empty and mcp_sendlist_empty images instead. So you'll have to create those two mcp master images, if you want them to be different from the regular mcp images.
REAPER Theme Development
https://www.reaper.fm/sdk/walter/images.php