Paste below CSS in your custom.css file

/* NINJA ------------*/
.nf-form-content input:not([type="button"]), .nf-form-content textarea {
    background: #fff!important;
    border: 1px solid #ddd!important;
}
.nf-form-content input.ninja-forms-field	{
  height: 45px!important;
}
.nf-field-label label {
    font-weight: 400!important;
}
.nf-form-fields-required	{
  padding: 10px 0 20px 0!important;
}
.nf-form-content input[type="button"]	{
  background: #333!important;
  padding: 10px 25px!important;
  border-radius: 4px!important;
  font-size: 16px!important;
  cursor: pointer!important;
}
.nf-form-content input[type="button"]:hover	{
  color:#fff!important;
  background: #666!important;
}
.nf-form-content .label-above .nf-field-label	{
  margin-bottom: 3px!important;
}
.nf-field-container {
    margin-bottom: 15px!important;
}