Welcome to the Theremin World forums Ftoso,
as usual with factory presets, some you will want to keep, but to edit/build your own and trying out other available settings made by others you can set up your own 'pallette' of sounds. There are not really direct 'emulations' of 'classical instruments', however, keeping in mind an advice for synthesizers in general, when you want to emulate a traditional instrument, study how that instrument is being played, and imagine you are playing that instrument while playing the synth or theremin.
Another aspect is matching the dynamic aspects, like a flute or brass wind instrument tends to have less vibrato (pitch hand) and more tremolo (voulume/expression hand), while for bowed strings it's the other way around, while a singing human voice can use both and vary the ratio between them.
The theremini is a hybrid instrument, a theremin front end (gesture control interface) operating a wavetable synthesizer with quite a lot of editing options whith the editor software/app. There is a learning curve in creating useful presets, but that mainly involves spending time with it and playing around and hearing and feeling what the various settings and parameters do, and saving the settings when you like it, and then after playing maybe tweak it some more.
While some call it 'a toy' outright, I think the Theremini does have it's place, and it's not the worse thing there is 'to get your fingertips wet'. Depending what music and playing style you're wanting to play on it, how you want to use it, where you are going with it, it may suffice, or you may get to the point in learning that you will want a more 'real/pure' theremin, or otherwise expand things with synth/synth modules, effect boxes, MIDI software etc. When you are seeking and want to experiment to see where it & you want to go, I've personally have found the Theremini to be a good starting point for that. But after less than a year I did buy an Etherwave model, as I felt I was starting to run into some of the Theremini's inherent shortcomings versus a pure analog traditional theremin.
Also take a look at this Animoog Love Theremini page