[Add Media] Caption Font Size

I have added a picture using Add Media with the caption option filled in.

[caption id="attachment_948" align="alignnone" width="320" font-size="8"]<img src="https://carolinayouthsymphony.org/wp-content/uploads/2015/02/600_9109a.jpg" alt="Back row Lee Street, Joseph Ritchie, Matthew Grant Harrison Thornton, Steven Chen, John Meza, Neil Gramopadhye Ayumi Chigusa, Karen Zhao, Stephan Voelk, Louise Averitt, Preston Lee J. Heeney, Maggie Azevedo, Sarah Coomes, Jalyn Haynes, Emmy Klaeser, Ikumi Chigusa McCullough Tarner, Madeline Corvin, Madison Bevan, Jayden Mae Hetzelberger Ansley Moe, Nick England, Brice Tiffany" width="320" height="256" class="size-full wp-image-948" /> Back row<br />Lee Street, Joseph Ritchie, Matthew Grant<br />Harrison Thornton, Steven Chen, John Meza, Neil Gramopadhye<br />Ayumi Chigusa, Karen Zhao, Stephan Voelk, Louise Averitt, Preston Lee<br />J. Heeney, Maggie Azevedo, Sarah Coomes, Jalyn Haynes, Emmy Klaeser, Ikumi Chigusa<br />McCullough Tarner, Madeline Corvin,<br />Madison Bevan, Jayden Mae Hetzelberger<br />Ansley Moe, Nick England, Brice Tiffany[/caption]

I have added following to css but the caption font size is not changing. What should I do?

.wp-caption .wp-caption-text {
font-size: 10px;
padding-top: 8px;
margin: 10px;
}