Newbie help with 3110 classic and .nth files

Hi all; first post on this group. Recently got myself a 3110 classic. I
downloaded some themes for it. These came in .zip files. Opened the files
and they then open as .nth files.
Nokia PC suite doesn't see the .nth files and wont let me transfer them
across.
A cut and paste in windows explorer allows me to transfer the files to the
phone memory and I placed them in Gallery> Themes.
Open the phone to select the theme to change it and the phone sees the theme
but it states 0 files under the theme heading and only gives the option to
'open' as opposed to 'apply theme'
What am I doing wrong?

Help please

TIA

Graeme