Since 2022.2, we have modified the UI Builder theme dropdown to show only what makes sense with the current document's "Editor Extension Authoring" setting.
In short: if this mode is enabled, we show all themes that are available in the Editor; if disabled, we show all themes that are available at runtime.
In the default mode (Editor Extension Authoring mode is not checked), only the Default Runtime Theme is available (plus any other custom theme), so you won't see Dark or Light themes, because these don't apply at runtime.