Hi,
while touching a theme styles I’ve found that the WP link dialog is affected by those style changes and some styles combinations hide the search input field.
That window was conceived to be shown just on the backend so it seems that some more styles are needed to enforce a consistent styling.
These are a good start for it:
.fl-builder #wp-link {
font-family: "Open Sans", sans-serif;
font-size: 13px;
}
.fl-builder #wp-link label {
margin-bottom: 0;
}
.fl-builder #wp-link input[type="text"] {
display: inline-block;
padding: 3px 5px;
}