Remove all the rounded corners in Bootstrap (restore to the default corner)

* {
  border-radius: 0 !important;
}

* {
  -webkit-border-radius: 0 !important;
     -moz-border-radius: 0 !important;
          border-radius: 0 !important;
}

variables.less

@border-radius-base:        0px;
@border-radius-large:       0px;
@border-radius-small:       0px;

In bootstrap 4 if you are compiling it you can disable radius alltogether in the _custom.scss file:

$enable-rounded: false;

CSS Vertical Alignment – Bootstrap Bottom Align within Row

.row {
position: relative;
}
.mainBox {
border: solid thin black;
}

.buttonBox {
position: absolute;
bottom:0;
right:0;
}

.buttonBox will make the div with the button align to the bottom of the container