You have the “mk–col–4-12” class container width set as 33.3%, so there will always be 3 columns no matter how small a screen. Add media queries that change this width when the screen is smaller than a certain width. One step can change the width to 50%, then another to 100%. For example:
@media screen and (max-width: 512px) {
.mk--col--4-12 { width: 50%; }
}
If you are not working on a child theme, the best place for custom CSS is tha additional CSS panel of the customizer.
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