code-lab/node_modules/vuetify/lib/components/VBtnToggle/VBtnToggle.css

.v-btn-toggle > .v-btn.v-btn--active:not(.v-btn--disabled) > .v-btn__overlay {
  opacity: calc(var(--v-activated-opacity) * var(--v-theme-overlay-multiplier));
}
.v-btn-toggle > .v-btn.v-btn--active:not(.v-btn--disabled):hover > .v-btn__overlay {
  opacity: calc((var(--v-activated-opacity) + var(--v-hover-opacity)) * var(--v-theme-overlay-multiplier));
}
.v-btn-toggle > .v-btn.v-btn--active:not(.v-btn--disabled):focus-visible > .v-btn__overlay {
  opacity: calc((var(--v-activated-opacity) + var(--v-focus-opacity)) * var(--v-theme-overlay-multiplier));
}
@supports not selector(:focus-visible) {
  .v-btn-toggle > .v-btn.v-btn--active:not(.v-btn--disabled):focus > .v-btn__overlay {
    opacity: calc((var(--v-activated-opacity) + var(--v-focus-opacity)) * var(--v-theme-overlay-multiplier));
  }
}
.v-btn-toggle > .v-btn.v-btn--active:not(.v-btn--disabled).v-btn--variant-plain {
  opacity: 1;
}