Theme

Your button's colors are defined under Site Settings, where you'll be able to select your primary and secondary colors. Styling buttons past this point, will require the use of inline styles or CSS applied to your buttons via Layout. In order to override MV's CSS you'll need to mark your values !important. 

https://w3schools.com has many excellent tutorials and examples of basic CSS editing.

Resources: https://www.w3schools.com/cssref/sel_hover.php 
 
Under Site Settings you'll see the Site Colors options which affects many elements.
 

 
Additionally, when adding buttons through the WYSIWYG, you can set additional styles.

MV uses the class, 'btn'.

.btn {
background-color: purple !important;
}

.btn:hover {
background-color: orange !important;
}