None of these answers seem to work for me, but this did:

.v-field.v-field--focused {
    i {
        color: #1976D2 !important;
    }

    .v-label {
        color: #1976D2 !important;
    }

    .v-field__outline {
        &__start, &__notch, &__end {
            color: #1976D2 !important;
        }
    }
}