To style the capture dialog box like in the example below, override some styles and options.

1. Custom styles.

   .dc-modal-content {
       border: none !important;
       border-radius: 15px !important;
   }
   
   .dc-modal-header {
       border-top-left-radius: 15px;
       border-top-right-radius: 15px;
       border-color: black !important;
       background-color: #19197e !important;
       overflow: hidden !important;
   }
   
   .dc-modal-header-title {
       color: white;
   }
   
   .dc-modal-body {
       background-color: #19197e !important;
   }
   
   .dc-canvas {
       background-color: black !important;
   }
   
   .dc-modal-footer {
       border-color: black !important;
       background-color: #19197e;
       border-bottom-left-radius: 15px;
       border-bottom-right-radius: 15px;
   }
   
   .dc-btn {
       background-color: black !important;
       color: white !important;
       border-color: black !important;
   }
   
   .dc-btn-light {
       border-color: darkgray !important;
   }
   
   .dc-btn-light:enabled:hover {
       border-color: red !important;
       background-color: #2d2d2d !important;
   }
   
   .dc-btn-main {
       background-color: green !important;
       border-color: green !important;
   }
   
   .dc-btn-main:enabled:hover {
       color: black !important;
       background-color: greenyellow !important;
       border-color: white !important;
   }
   

2. Custom options for captureSignature() method.

   var options = {
       strokeColor: 'red',
       html5CaptureOptions: {
           captureDialogWidth: 700,
           captureDialogHeight: 400,
           captureDialogTitle: 'Draw your signature',
           showSignatureLine: true,
           signatureLineText: 'Please sign here',
           decorColor: 'yellow',
           showClose: false
       }
   }