Thanks for your clarification about the tooltip: actually we would like to hide the tooltip (and thus the number), not just the number (i.e. your first screen). May I ask you some indications about how to customize the "custom.css" file?
My understanding is that I should:
1) access the administration/themes section
2) select a theme and "extend" it (I guess this is to duplicate the theme, in order to avoid changes on the default ones); e.g. "vanilla", and I rename it in "extends_vanilla_TEST"
3) then, in the "theme editor" for "extends_vanilla_TEST" I open the custom.css file, I add your lines (like shown below?) and save the file ("Copy into local theme and save changes")
4) if this is correct, how can I force my survey to use the "extends_vanilla_TEST"theme?
Thanks a lot, best regards
nicola

/******************
User custom CSS

---

In this file you can add your own custom CSS
It will be loaded last, so you can override any other property.
Also, it will never be updated. So if you inheritate a core template and just add here some CSS, you'll still benefit of all the updates
*/
.slider .tooltip.top {
display:none;
}

That's the way to do it. Exactly.

Joffm

I guess this is to duplicate the theme, in order to avoid changes on the default ones

Not only this, but there is the heritage.
E.g. If you extended one theme, set some basic styles there which you would like to have for all your themes, then extend again to set some specific styles (corporate design).
Now, if you change something in your basic styles it is herited from the next extensions.

Joffm

if this is correct, how can I force my survey to use the "extends_vanilla_TEST"theme?

After extending, the new theme will be available in your survey General settings.