Hello,
Please add this code to the Theme Settings > Custom CSS > Global:
body .wd-sizeguide-btn .wd-open-popup {
color: var(--wd-primary-color);
position: relative;
display: inline-flex;
align-items: center;
justify-content: center;
outline: none;
border: 2px solid var(--wd-primary-color);
border-radius: 0;
box-shadow: none;
vertical-align: middle;
text-align: center;
text-decoration: none;
text-transform: var(--btn-transform);
text-shadow: none;
font-weight: var(--btn-font-weight);
font-family: var(--btn-font-family);
font-style: var(--btn-font-style);
line-height: 1.2;
cursor: pointer;
transition: color .25s ease, background-color .25s ease,
border-color .25s ease, box-shadow .25s ease, opacity .25s ease;
padding: 5px 14px;
min-height: 36px;
font-size: 12px;
}
body .wd-sizeguide-btn .wd-open-popup:hover {
color: #FFF;
background-color: var(--wd-primary-color);
}
Best Regards