Issue:

The text for the onlabel and offlabel is not shown because the ".k-switch-label-on, .k-switch-label-off" is showing "display: none" and other issues you'll see when testing.  Appears that the SASS Theme Builder doesn't take into consideration the OnLabel and OffLabel parameters.

SASS Theme CSS: line 12087-12096

.k-switch-label-on,
.k-switch-label-off {
    display: none;
    position: absolute;
    top: 50%;
    -webkit-transform: translateY(-50%);
        -ms-transform: translateY(-50%);
            transform: translateY(-50%);
    overflow: hidden;
}

Code:

<TelerikSwitch class="ml-2" @bind-Value="@isDark" Width="80px" OnLabel="Dark" OffLabel="Light" OnChange="ThemeHandler" />