.row_1 {
  max-width: 900px;
  margin: auto;
}
/* Clear floats after the columns */
.row_:after {
  content: "";
  display: table;
  clear: both;
}

/* VOOR MOBIEL: EEN KOLOM_*/
.column_{
  float: left;
  width: 100%;
  padding: 8px 5px;
}
.TekstOnder_{
  color: #00ff00;
  text-align: center;
  font-size: 3.5vw;
  padding: 0px;
  background-color: none;
}

<!---->  
/* Responsive layout - TWEE KOLOMMEN */
@media screen and (min-width:600px) {
  .column_{
   width: 50%;
  }
  .TekstOnder_{
   color: #00ff00;
   text-align: center;
   font-size: 1.6vw;
   padding: 0px;
  }
} 
<!---->  
 /* Responsive layout - Twee KOLOMMEN Vast*/ 
@media screen and (min-width:900px) {
  
  .TekstOnder_{
   color: #ff0000;
   text-align: center;
   font-size: 15px;
   }
}