Hello Stephan,

Which button exactly? if we are talking about 'Download' button in this page:

http://clapat.ro/themes/soho-wordpress/soho_portfolio/minimal-vibes/

.clapat-button {
    color: #fff;
    background-color: #000;    
}

.clapat-button:hover, .clapat-button:active, .clapat-button:focus {
background-color:#333;
color:#fff;
}

change the colour values and add the code to  theme options -> styling options -> custom CSS