u can use css snippet below. just change #000000 to ur preferred color. in case u r using on top of community theme and it has a custom hr line color, add !important between the color code and the semi colon ; i.e. #000000 !important;
the
hrseems repeated in the css below because of reading view and editing view
to apply the css snippet, refer to link to obsidian help below (basically u need to save this in a text file with .css extension and save it in .obsidian/snippets folder.
.markdown-preview-view hr, hr {
border-color: #000000;
}