There is so parameter that could be passes to change the color of the sliderInput as per the docs. But you can html css formatting. something like this:
tags$head(tags$style(HTML('.js-irs-0 .irs-single, .js-irs-0 .irs-bar-edge, .js-irs-0 .irs-bar {
background: #000069;
border-top: 1px solid #000039 ;
border-bottom: 1px solid #000039 ;}
/* changes the colour of the number tags */
.irs-from, .irs-to, .irs-single { background: #000069 }'
))
)