The files, (dark-bg and BG) are currently in .jpg format. Maybe the problem is only on your pc. I checked it on several computers and did not see an error, plus some friends also checked.
The names are a "nod" to the (Darling in the FranXX) series.
(愛, Ai):Love, (ココロ, Kokoro):Heart, (ダーリン, Darling)
If you have not seen the series, you will not understand the references of the names, also (Love, Heart and Darling) is related to the song.
You mean it because of the heart-shaped sliders? Well in the first updates the sliders were a circle but at the request of some people they told me to change them to a heart shape.