The module name should not be passed as a hard-coded value

The module name should not be passed as a hard-coded value, since it is not a valid theme name. Refs. https://github.com/zikula/core/issues/1816

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -
This value was `Theme` in Core 1.3.3 and `ZIkulaThemeModule` in Core 1.3.5+ I'm not sure this is the source of the problem as module upgrades have worked in those releases.