First, extend the theme -
manual.limesurvey.org/New_Template_Syste...S3.x/en#Theme_Editor
If using the fruity/skyline-blue variation and you want to restrict this to yes/no questions, add something like this to the end of the
custom.css file:
Code:
.yourThemeName div.yes-no .btn-check:checked + .btn-primary {
background-color: #00A9F8;
border: 1px solid #00A9F8;
}