/* This overrides the primary color for the 'light' scheme */
[data-md-color-primary="custom"] {
  --md-primary-fg-color:        #A50000;
}

/* If you use the header/tabs, you might also want to set the text color 
   to ensure it's readable against your dark red */
[data-md-color-primary="custom"] .md-header {
  background-color: #A50000;
}