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The markings on the thermostat say "LINE RED" and "LOAD BLACK". Does that mean one red wire on the thermostat goes to hot white in the box and the other red wire on the thermostat goes to hot black in the box, and one black wire in the thermostat goes to load black in the box and the other black wire in the thermostat goes to load white in the box?
In this case, the hot wires are on the right-hand side of the box per voltage tester.
Breaker switch is now in the off position.
However they are used for all tasks. One way is "/2" with black, white, bare wires. The other is "/3" with black, white, red, bare wires. This is how cables are made. As such, wire colors tend not to reflect the actual job the wire is doing.
Often, neutral is not needed in a circuit because of the way American 120/240V works. In that case, only 2 wires are needed (no neutral). Yet, when you go to the shop and try to buy /2 cable, it only comes black-white, so that's what you use.
Neutral.... white or gray
Ground.... green, yellow/green, or bare
Hot.... all other colors
If neutral is not present, as in here, then white can be re-used for a hot wire. In the 2000s Code was changed to require white wires used as hots to be marked a "hot color" with tape or paint. Any hot color will do. They should be marked at both ends.
As far as how to wire your particular thermostat, you'll need to read the instructions. NEC 110.3(B) requires you to follow the instructions when installing equipment.
