You can try this.
Replace the #Selector ID with border ID using inspect element then use it.
#Selector ID{
broder: 2px solid transparent !important;
}

Hi Caroline92 ,
You can certainly do that by using CSS border property to set its value to "0px""
For example: .classnameof borderelement{
border: 5px solid red;
}
Replace above by:
.classnameof borderelement{
border: none !important;
}.
For further details please share preview link of your survey, to better guide you(of that particular page where it happens)

https://community.qualtrics.com/XMcommunity/discussion/comment/54460#Comment_54460Preview - Qualtrics Survey | Qualtrics Experience ManagementThank you very much for your reply! This is the preview link. I tried some code, still couldn't remove the black bolder. Please kindly have a further look. Thank you!

Caroline92
Kindly request you to add below properties to you code and try ,it works at my end for your code
.classnameofbutton:focus{
outline:0;
box-shadow:0
}.

https://community.qualtrics.com/XMcommunity/discussion/comment/54454#Comment_54454Thank you so much for your help!

https://community.qualtrics.com/XMcommunity/discussion/comment/54478#Comment_54478Thank you so much for your guidance!!! Finally, I am able to remove the border color. Cheers!!!
.Skin #Buttons #NextButton:focus,.Skin #Buttons #PreviousButton:focus{background:#1779BB;outline:none;box-shadow:none;}