In whatever theme CSS you are using you would need to find .markdown-preview-view hr. That contains a border-color. In my theme that variable was var(--background-modifier-border) but you can change that, either to another variable, OR if you don’t know what all the variables look like (I don’t either, but depending on your code IDE you can see them visually represented within the CSS file) you can also set it to a regular color value, such as #ff9900 for bright orange or #0099ff for a blue line.
So, it could look something like this in the CSS:
.markdown-preview-view hr {
border: none;
border-top: 1px solid;
border-color: #ff0000; /* Bright red */
}