mempool/frontend/src/app/components/theme-selector/theme-selector.component.html

<div [formGroup]="themeForm" class="text-small text-center">
    <select formControlName="theme" class="custom-select custom-select-sm form-control-secondary form-control mx-auto" style="width: 150px;" (change)="changeTheme()">
        <option value="default" i18n="theme.mempool-theme">Classic</option>
        <option value="contrast" i18n="theme.high-contrast">BlueMatt</option>
    </select>
</div>