At least I figured out that the button can also be shown as a normal link. Here is my CSS I use, maybe someone will need it.
.text_modal {
display: initial !important;
line-height: normal !important;
background-color: transparent !important;
text-decoration: underline !important;
border: 0 !important;
padding: 0 0 1px 0 !important;
margin: 0 !important;
}
.text_modal span {
color: #green;
border-bottom: solid 1px #green;
text-decoration: none;
font-weight: normal;
font-size: 14px;
}
.text_modal span:hover {
color: #yellow;
border-bottom: solid 1px #yellow;
text-decoration: none;
}
And in the text editor I use <div style="p {display: none;">Content in here...</div>
as WP seems to add a </p>
right after the implementation of the modal button, which is kind of weird.