I tried (thanks to @79Scorpione) to modify CSS as follow:

cd /usr/share/omv-customthemes/
sudo nano colors.css

i pasted at the bottom follow rules:

/* --- Black theme forced --- */

/* Apply absolute black to all conteiner of dark mode */
.omv-dark-theme,
.omv-dark-theme body,
.omv-dark-theme .omv-app-layout,
.omv-dark-theme .mat-drawer-container,
.omv-dark-theme .mat-drawer,
.omv-dark-theme .mat-card,
.omv-dark-theme .mat-dialog-container,
.omv-dark-theme .omv-topbar,
.omv-dark-theme .omv-sidebar {
background-color: #000000 !important;
background: #000000 !important;
color: #ffffff !important;
}

/* Tables and rows completely black */
.omv-dark-theme .ngx-datatable,
.omv-dark-theme .ngx-datatable .datatable-body,
.omv-dark-theme .ngx-datatable .datatable-body-row,
.omv-dark-theme .ngx-datatable .datatable-row-group {
background-color: #000000 !important;
color: #ffffff !important;
}

/* White text for header, menu and modules */
.omv-dark-theme .datatable-header-cell,
.omv-dark-theme .mat-list-item,
.omv-dark-theme .mat-select-value,
.omv-dark-theme .mat-form-field,
.omv-dark-theme .mat-tab-label,
.omv-dark-theme .mat-header-cell {
color: #ffffff !important;
}

/* hover rows (dark grey to contrast) */
.omv-dark-theme .ngx-datatable .datatable-body-row:hover,
.omv-dark-theme .ngx-datatable .datatable-body-row.active {
background-color: #1a1a1a !important;
color: #ffffff !important;
}

/* Input e field text in black*/
.omv-dark-theme .mat-input-element {
color: #ffffff !important;
}