How to remove rectangular border from input field?

Hello, 

I've been trying to remove this blue rectangular border for a while now. I have tried everything like:

.nf-section {

border: none;

}

input {

border: 0!important;

}

I have even tried removing it from the Advanced Settings of the control > Border. I tried changing the color as well as the size, but nothing works. 

If the border:none; CSS is applied, it removes the inner blue border that you can see on the image attached to this post. Nevertheless, I have no problem with the inner border. I want to get rid of the outer rectangular border. 

Anyone has any idea how I can do that?

Thanks!