Please go to My Site → Customize → CSS (or Additional CSS), then add all of the following code to the bottom of that window.
/* This CSS changes the color of the captions on the sidebar gallery.
* 11442802-hc (jw)
*/
#media_gallery-9 .slideshow-slide-caption {
color: #ff0;
font-size: 14px;
text-shadow: #222 1px 1px 2px;
}
Tip: keep the /*
comment */
in the code to later know what this code does.