This is where most people fail. You cannot install a Symbian Belle theme on Symbian Anna.
If you download a theme and it fails to work, you are likely encountering one of three common issues: nokia c7 themes work
The most critical aspect of finding a theme that "works" is understanding the operating system version. The Nokia C7 launched with Symbian^3 but was later upgradable to Symbian Anna and eventually Nokia Belle. This is where most people fail
Most themes designed for Symbian^3 (S^3) will work on the C7. However, if you have updated your phone to Nokia Belle (the final major update), the landscape changes slightly. The Nokia C7 launched with Symbian^3 but was
Why? The theme was designed for Symbian Anna, but you are using Belle (or vice versa). Belle FP2 uses a different icon layout (six icons in the pull-down status bar, etc.). Fix: Search specifically for "Belle FP2 themes" if you updated your C7 to Belle Feature Pack 2. Alternatively, install a theme patch using RPPlus or RomPatcher+ (advanced users only).
Nokia C7 themes are packaged as SIS (Software Installation Script) or SISX files. These are essentially compressed archives containing: