Leave the style settings in appearance at 'Use theme styles'.

/* -- START Hbook WPhotel -- */
/* Hbook input fields */
.hbook-wrapper input[type="text"], .hbook-wrapper
input[type="number"], .hbook-wrapper select, .hbook-wrapper
textarea {
border: 1px solid #CDBAD6 !important;
padding: 6px 12px !important;
border-radius: 3px !important;
height: 40px;
}
.hbook-wrapper input[type="text"]:focus, .hbook-wrapper
input[type="number"]:focus, .hbook-wrapper select:focus,
.hbook-wrapper textarea:focus {
box-shadow: 0 0 0 #CDBAD6,0 0 8px #CDBAD6!important;
}
/* Hbook buttons */
.hbook-wrapper input[type="submit"] {
background: #cdbad6 !important;
border: 1px solid #696298 !important;
font: inherit !important;
font-size: 20px !important;
color: #696298 !important;
padding: 6px 12px !important;
border-radius: 3px !important;
}
.hbook-wrapper input[type="submit"]:hover {
background: #696298 !important;
border: 1px solid #cdbad6 !important;
color: #cdbad6 !important;
}
/* - available but not selectable background color - */
.hb-dp-day-not-selectable:not(.hb-day-fully-taken) {
background: #c0f7c0 !important;
}
/* -- END Hbook WPhotel -- */
Code language: CSS (css)