These files are created by the Nokia Theme Studio application, which is used to create themes for your Nokia cell/mobile phone.
A mobile phone theme is a special file which may include pictures, images, icons and ring-tones enabling you to personalise the look of your phone.
NTH files are used for phones based on the S40 (series 40) platform, phones which utilise this platform include: Nokia 6230i, 6300, 6125, etc.
NTH themes can be downloaded to your PC via the internet, which then need to be transferred to your phone, this can be done using blue-tooth or by transferring the file into your phones memory using a USB card reader or USB data cable. (Use your computers file-browsing software to drag it across as if to any other form of data storage i.e. another hard-drive).
Once the theme has been transferred into your phone simply select it from the gallery and apply.
* You do not need to open NTH files on your PC (just transfer to any phone which supports the format as shown above).
** NTH files may be downloaded in .ZIP format or similar - if this is the case unzip the file using WinZIP, WinRAR, etc. before following the steps listed above.
*** NTH files are not compatible with Nokia phones based on the S60 platform which use .SIS files.