How to change the default blockquote colors

Add the following custom CSS code in Theme Options > Styling > Custom CSS > Custom CSS:

.entry q,
blockquote {
 background:#eee;
 color:#000000
}

/* Icon Color */
.entry  q::before,
.entry blockquote::before {
 color:#ff0000;
}

/* links */
.entry q a,
.entry blockquote a {
 color: #0000ff;
}
.entry q a:hover,
.entry blockquote a:hover {
 color:#000000;
}

/* Author Name */
.entry q cite,
.entry blockquote cite {
 color:#ff00ff;
}

Make sure to clear both, your browser's and website's cache after saving Theme Options and before viewing changes to your website https://jannah.helpscoutdocs.com/article/12-how-to-clear-cache