Hi Jemma,
this is happening because text color is set to white color and background is also white.
So, to set black color to button text we open your style.css
(you can find it here \wp-content\themes\andy\style.css or through WordPress editor)
on line 2652 you have this code:
.load_more_portfolio, .more_portfolio_link a {
color: #ffffff!important;
font-size: 16px;
font-weight: 700;
border: 2px solid #b9b9b9;
text-transform: uppercase;
padding: 26px 91px 26px 90px;
display: inline-block;
cursor: pointer;
-webkit-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;
}
We replaced color: #ffffff!important;
to color: #000!important;
or you can set some other color that you like :)
Cheers