Here is a list of sass variables you can set in your admin themes.
Note
Remember that bootstrap also defines many variables you can customise.
Perform uses these variables throughout, so any changes you make to
them will cascade across Perform styles too.
Look for _variables.scss in the bootstrap source for the complete list.
Background color of hovered datepicker cells.
Default value: gray('200')
Color of datepicker cells not in the current range.
Default value: gray('400')
Color of hovered datepicker cells not in the current range.
Default value: gray('200')
Background color of hovered datepicker cells not in the current range.
Default value: gray('500')
Default value: #cdcdcd