Solved, added the css below :
/* color and textsize for gallery caption bottom*/
.title-display-slideup-stick a .photonic-title {
font-size: 1em;
color: whitesmoke ;
}
/* text color and size in photoswipe */
.pswp * {
color: red;
font-size: 18px;
}
/* color and textsize for gallery caption below */
a.photonic-lb.photonic-photoswipe.photoswipe {
text-decoration: none;
color:black;
text-align: center;
font-size: 1.2em;
}