photoprism/frontend/src/css/themes.css

.theme-onyx .v-content__wrap,
.theme-onyx .p-page,
.theme-onyx .form,
.theme-onyx .v-content {
    background: #525252 !important;
}

#photoprism.theme-onyx .theme--light.v-small-dialog__content,
#photoprism.theme-onyx .theme--light.v-sheet,
#photoprism.theme-onyx .theme--light.v-card {
    background: #525252;
}

.theme-onyx .application.theme--light {
    background: #525252;
}

#photoprism.theme-onyx .theme--light .v-table {
    background: #4E4E4E;
}

#photoprism.theme-onyx .theme--light.v-table thead th,
#photoprism.theme-onyx .theme--light.v-table tbody td {
    color: #fff;
}

#photoprism.theme-onyx .theme--light.v-table tbody tr:hover {
    background: #666;
}

#photoprism.theme-onyx .theme--light.v-chip,
#photoprism.theme-onyx .v-card__actions .theme--light.v-text-field--solo>.v-input__control>.v-input__slot {
    background: #777;
}

#photoprism.theme-onyx .theme--light.v-text-field--solo>.v-input__control>.v-input__slot {
    background: #4E4E4E;
}

#photoprism.theme-onyx,
#photoprism.theme-onyx .p-page a,
#photoprism.theme-onyx .v-datatable a,
#photoprism.theme-onyx .theme--light.v-expansion-panel .v-expansion-panel__container,
#photoprism.theme-onyx .theme--light.v-tabs__bar .v-tabs__div,
#photoprism.theme-onyx .theme--light {
    color: #fff;
}

#photoprism.theme-onyx .theme--light.v-list {
    background: #555;
}

#photoprism.theme-onyx .theme--light.v-select .v-select__selections {
    color: #e3d5c3;
}

#photoprism.theme-onyx .theme--light.v-list .v-list__tile__sub-title,
#photoprism.theme-onyx .accent--text {
    color: #e3d5c3 !important;
}

#photoprism.theme-onyx .p-page a.text-link {
    color: #e3d5c3;
}

#photoprism.theme-onyx .theme--light.v-input:not(.v-input--is-disabled) input,
#photoprism.theme-onyx .theme--light.v-input:not(.v-input--is-disabled) textarea {
    color: #fff;
}

#photoprism.theme-onyx .theme--light.v-btn.v-btn--disabled,
#photoprism.theme-onyx .theme--light.v-btn.v-btn--disabled .v-btn__loading,
#photoprism.theme-onyx .theme--light.v-btn.v-btn--disabled .v-icon {
    color: #999 !important;
}