In the past, I unintentionally discovered that Cyanogenmod 9 can use CM 10 themes.
While the play store shows CM 11 themes as incompatible on CM 10, would the themes work if installed manually?
The new Theme Engine, introduced with CyanogenMod 11 and later adopted by other KitKat custom ROMs, uses a different but simpler structure from past engines.
The theme files are all under the
assets folder of CM11 theme APKs, while it was solely up to the developer as to where to put the theme files in the old Theme Engine themes.
While I have seen that there are some backwards-compatible themes available, they generally specify the target CM versions in the Play Store app listings. So if Play Store tells you that your device is incompatible, better heed the warning; but if you see in the app description / reviews that people using CM < 11 can install the theme, then maybe you can install the theme manually.
Try to see if people using CM < 11 have successfully used the theme. If so, then go ahead!