@cph5489 welcome to the forum.
In userstyle.css you could try adding:
blockquote {
opacity: 1;
/* changes the blockquote text from the default of 0.7 */
}
and I believe that if you want to change the border as well it would be:
blockquote {
border-left: 4px solid #FFFFFF;
/* sets the colour of the blockquote border */
opacity: 1;
/* changes the blockquote text from the default of 0.7 */
}