In the dark theme i can see that after certain usage of the app the surface feels as if it is hot after too much use. When does it happen?

Also what did Skritter team mean when they did the teaser video saying 'Let the sparks fly' ?

The sparks happen when you get a lot of characters right in the row. The more you get right the more sparks appear, but if you make a mistake they disappear. It's just a nice effect that's encouraging - if you see lots of sparks your doing well and not making mistakes!

"Let the sparks fly" seems like just a reference to studying well - "You have the app, so study well and the sparks will fly".

"sparks fly" is an idiom that is used when something is progressing well, and is exciting.

From the Complete Oxford English Dictionary:

f.1.f Phr. sparks fly and varr.: heated words are spoken, friction or excited action occurs.

   [...] 1977 Western Morning News 1 Sept. 10/3 Robertson scored from the spot, and then sparks really began to fly.

Xie xie guys

I have not seen the video, but believe this is for the iOS app.

I have to say that originally, I did not like the sparks effects. Thought it was a bit hokey, a bit too much. However, I have since changed my opinion on this, and find the effect pretty cool, not overdone, and complementary to the audio sound effects. The one thing I find a little unnerving is the voice that comes on, marking milestones in the time of your current study session. I would prefer that to just be a soft tone or gong or something along those lines.