jellyfin-web/dashboard-ui/devices/android/android.css

paper-tab, paper-tab a {
    font-weight: bold !important;
}

.libraryViewNav .pageTabButton {
    font-weight: bold !important;
}

    .libraryViewNav .pageTabButton:focus {
        font-weight: bold !important;
    }

@media all and (min-width: 300px) {

    .libraryViewNav {
        display: flex;
    }

        .libraryViewNav .pageTabButton {
            flex-grow: 1;
        }
}