Change the styles of the Bootstrap form fields

add a class form-personalizado where you want it to have transparent background and border and in css add that

.form-personalizado{
    border-bottom: 1px solid #00ff00;
    background-color:transparent;
}

NOTE: Your input must have the form-custom class or will not work (% with%)

You can use these examples below on this page of the Bootstrap documentation, it has a specific class for transparent background.

link

<button type="button" class="btn btn-outline-primary">Primary</button>
<button type="button" class="btn btn-outline-secondary">Secondary</button>
<button type="button" class="btn btn-outline-success">Success</button>
<button type="button" class="btn btn-outline-danger">Danger</button>
<button type="button" class="btn btn-outline-warning">Warning</button>
<button type="button" class="btn btn-outline-info">Info</button>
<button type="button" class="btn btn-outline-light">Light</button>
<button type="button" class="btn btn-outline-dark">Dark</button>