Try to find where ".survey-question-text" can be found in your source code.

Usually the description should be found quite next to it.

okay, I got it now. The solution is:
.survey-groupdescription p {
margin-top: 8px;
}

.survey-groupdescription p {
margin-bottom: 8px;
}

Top has also to be influenced. That is why the distance could be decreased only down to a special limit before.
But wouldn't it be a good idea to have that parameter as a simple button in the design editor as well??...

May I ask you...
Could you please be so kind and have a short look to my article "General data management with LimeSurvey not clear" as well? (It is very important for me to finish my survey by that!)
I would wish to understand things around participants, token table, etc...

Thank you VERY much!!

The editor is a standard tool included into Limesurvey. So I don't think it is that easy to "add a simple button".

Additionally, I think this should be organized in the template and not every survey editor should be able to do this. So my answer would be "No".