Well, on one page (home) i like change style of buttons (generated with inspector) and I use child theme
- Type -> transparent
- Shape -> square
- Size -> Regular
- class my-black-button
My Global CSS
.post .entry-wrap {
background-color: #fff;
box-shadow: none;
}
article.comment {
    background-color: #fff;
    box-shadow: none;
}
.x-colophon.top {
  background-color: #383838;
}
.x-colophon.bottom {
  background-color: #383838;
}
.x-block-grid-item h4, .x-block-grid-item h6 {
margin-top: 0px !important;
text-align: center;
color: #ffff;  
}
.white {
  color: #ffffff !important;
}
.greyblack {
  color: #383838
}
.greylight {
  color: #cacaca
}
.redsna {
  color: #760300
}
.black {
  color: #000000
}
.my-black-button {
  background-color: black;
  color: black;
  border-color: black;
}
.my-black-button:hover {
  background-color: white;
  color: white;
  border-color: white;
}
[type="submit"] {
    background: none;
    border-radius: 0px;
    box-shadow: none;
    font-size: 19px;
    color: black;
    text-shadow: none;
}
select, textarea, input[type="text"], 
input[type="password"], input[type="datetime"], input[type="datetime-local"], input[type="date"], input[type="month"], input[type="time"], input[type="week"], input[type="number"], input[type="email"], input[type="url"], 
input[type="search"], input[type="tel"], input[type="color"], .uneditable-input {
    background-color: #f9f1f1;
}
.contactFormSendButton {
  margin-bottom: 0;
  border-width: 3px;
  text-shadow: none;
  text-transform: uppercase;
  background-color: transparent;
  box-shadow: none;
  border-radius: 0;
  padding: 0.563em 1.125em 0.813em 1.125;
  font-size:16px;
}
This made according https://theme.co/apex/forum/t/customizations-implementing-additional-button-colors/206 and I tyr several options, such on css, .x-btn.my-black-button and x-btn.my-black-button:hover, and others.
But not work… also now I see a original button instead transparent button.
My site is : http://seguimundonavarro.com