How to i adjuste the flexboxes size i have done them but i need 1 more large flexbox around the flexboxes i have done for backround so what i mean is minimize the flexboxes and set 1 large light brown

How to i adjuste the flexboxes size i have done them but i need 1 more large flexbox around the flexboxes i have done for backround so what i mean is minimize the flexboxes and set 1 large light brown flex box for background i am stuck on this question. The flexboxes need to be different sizes

       Myndnotkun eftir sjástærð |

       myndnotkun |

       flexbox uppsetning |

       Html Grindakerfi |

html,body{

   height:100%;   

   margin:0;

   padding:0;

   border:0;

}

div{

   margin:0;

   border:0;

}

textarea {

   margin:0;

   border:0;

   padding:0;

   height:100%;

   width:100%;

}

.content{

   display:table;

   width:100%;

   border-collapse:separate;

   height:80%;

}

.Col{

   display:table-cell;

   width:30%;

   height:100%;

}

#header,#footer{

   height:10%;

   position:relative;

   z-index:1;

}