Please help us help you and fill where relevant:
Your LimeSurvey version: [see right hand bottom of your LimeSurvey admin screen] 6
Own server or LimeSurvey hosting: LImesurvey.org Hamburg
Survey theme/template: fruity
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Have two probably easier questions: 
1) I would like the whole survey including buttons to appear in simple black and white. 
2) I would like to replace the text on the NEXT button by "ENTER"
 

Hi,
1. If you want to use "fruity", extend the theme and adapt the colours to your wishes.
Maybe you may find a different theme better as the base theme.
"bootswatch" is nearly black and white,
"vanilla" is the easiest to adapt.

2.
See
forums.limesurvey.org/forum/can-i-do-this-with-limesurvey/130236-customizing-buttons-in-the-end-message-of-a-survey#235728

Thank you for your quick answer. I switched to Bootswatch in the survey. However, it seems, I cannot save the change f theme. Is there anything in particular I have to do?
Would I change the text of the NEXT button in the theme as well?

In fruity you can actually change the colour to black and white under theme options, then you go down to box "variations" and choose black pearl.