https://wordpress.org/plugins/video-conferencing-with-zoom-api/ style changes for meeting single page.

video-conferencing-zoom-api.css

//Copy to your css file

//For changing Countdown cell background
.dpn-zvc-single-content-wrapper .dpn-zvc-sidebar-wrapper .dpn-zvc-timer .dpn-zvc-timer-cell {
  background: #color_code; //For box background color
  color: #color_code; //For text color
}

//For changing color of DETAILS section
.dpn-zvc-single-content-wrapper .dpn-zvc-sidebar-wrapper .dpn-zvc-sidebar-tile {
   background: #color_code; //For box background color
}

.dpn-zvc-single-content-wrapper .dpn-zvc-sidebar-wrapper .dpn-zvc-sidebar-tile h3 {
  color: #color_code; //For text color
}

//Change background color and text color for join links 
.dpn-zvc-single-content-wrapper .dpn-zvc-sidebar-wrapper .dpn-zvc-sidebar-box .join-links .btn-join-link {
  background: #color_code; //For box background color
  color: #color_code; //For text color
}

//Change background and text color for start link
.dpn-zvc-single-content-wrapper .dpn-zvc-sidebar-wrapper .dpn-zvc-sidebar-box .join-links .btn-start-link {
  background: #color_code; //For box background color
  color: #color_code; //For text color
}

//Change background and color for meeting started and ending box
.dpn-zvc-single-content-wrapper .dpn-zvc-sidebar-wrapper .dpn-zvc-timer .dpn-zvc-meeting-ended {
  background: #color_code; //For box background color
  color: #color_code; //For text color
}