vue-material/src/components/mdCheckbox/mdCheckbox.theme

.THEME_NAME {
  .md-checkbox,
  &.md-checkbox {
    &.md-checked {
      .md-checkbox-container {
        background-color: PRIMARY-COLOR;
        border-color: PRIMARY-COLOR;
      }

      .md-ink-ripple {
        color: PRIMARY-COLOR;
      }
    }
  }
}