You may need to enable installation from "All sources" in Settings > Application manager > Installation settings to install third-party themes. Always scan files with antivirus software, as some archived Symbian files may contain malware.
So charge that old battery, blow the dust off the micro-USB port, and start downloading. Your E5 is waiting for a makeover. nokia e5 themes
This handbook explains Nokia E5 themes: what they are, how they work, how to create, install, customize, distribute, troubleshoot, and preserve them. It covers tools, file formats, design guidelines, technical constraints, step-by-step workflows, examples, and best practices for developers, designers, and enthusiasts working with the Nokia E5 (Symbian S60 3rd Edition FP2) theme system. You may need to enable installation from "All