It is dark / light theme sensitive.
.markdown-preview-view blockquote {
border-color: transparent;
font-size: 95%;
text-align: center;
hyphens: auto;
word-break: keep-all;
color: var(--text-muted); # you need to define
font-color: #aacdbe;
line-height: 1.3;
padding: 1px 2% 1px 2%;
margin-top: 15pt;
margin-bottom: 15pt;
}