Those mats don't need a thermostat because they only get so warm. The temperature of your soil will vary dependant upon the ingredients of the mix, moisture content, etc. For most seeds, the temperature of the medium does not have to be exact; those mats are far from exact. You might want to get a soil thermometer, however, to set your mind at rest.

I use heating cables, which do have a thermostat. They keep the medium at about 74 degrees, perfect for seed germination.

Here is a link that might be useful: {{gwi:211854}}