hello, if I change the color drop-down menu the value is reset to 25%, I downloaded style.css after the color change and I changed the value to 31% but I want to avoid that I have to do the same procedure when changing a color

You should re-apply my hack after you apply the them designer, OR you put my code above on another css file, and put tag !important on it :

.joms-list--thumbnail .joms-list__item {
  float: left;
  width: 31% !important;
  padding: 2px;
}

This problem will be fixed with the next update?

actually not, I think our dev forgot to solve this on newest version, for time being once you update your jomsocial please you apply again my solution above