Hi Huzma,
If you open your app in chrome and right click thumb - choose Inspect Element - from here you can see the .css classes and files.
In your case look at .claro .dijitSliderImageHandle and dijitSliderImageHandleH - override these in your own .css
claro:
http://js.arcgis.com/3.14/dijit/themes/claro/claro.css
.claro .dijitSliderImageHandleH
As an example: in Launchpadtheme for web app builder this style is overrided:
overrided claro:
themes/LaunchpadTheme/styles/default/css/dojo-override.css
.claro .dijitSliderImageHandle