For Demo

• Sign in to the Blogger Account
• Select Template and click Edit HTML (Click on Proceed)
• Find ]]></b:skin> (Find using ctrl + F) [Alternate Wayt to copy css code]
• Copy any of  the type of below css code  just above it.
• Click on Save

Comments Style Version 1.0

.comments .comment .comment-actions a {background: none repeat scroll 0 0 #FFD99E;border: 2px solid #FFB43D;border-radius: 3px 3px 3px 3px;color: #000000;font: bold 12px arial;margin-right: 14px;padding: 3px 9px;text-decoration: none !important;text-shadow: 1px 1px 0 #FFFFFF;
}
.comments .comment-block {
background: none repeat scroll 0 0 #E2E2E2;border: 4px solid #ABABAB;border-radius: 3px 3px 3px 3px;padding: 3px 10px;
}
.continue a {background: none repeat scroll 0 0 #FFD99E;border: 2px solid #FFB43D;border-radius: 3px 3px 3px 3px;color: #000000 !important;display: inline-block !important;margin-top: 7px;padding: 3px 8px !important;text-decoration: none !important;text-shadow: 1px 1px 0 #FFFFFF;}.comment-block:hover > .comment-header {border-bottom: 2px solid #000;-moz-transition: border-color .5s ease;
}
.comment-header a {
color: #000000 !important;
}
#comments .avatar-image-container img {border: 2px solid #FFFFFF !important;border-radius: 50px 50px 50px 50px;height: 57px;max-width: 57px;
}
.comments .avatar-image-container {border: 2px solid #FFFFFF;border-radius: 34px 34px 34px 34px;box-shadow: 1px 1px 3px #ABABAB;float: left;margin-left: -40px;max-height: 60px !important;overflow: hidden;width: 60px !important;} 

Comments Style Version 2.0

.comments .comment-block {background: url("https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5Mocf2M-F_z2Gyvi1h0kY2pojfpYw9m29LGHXJ8z5SIIVbPCU-_m7YaWrShrQ2xL_SsyA3lRkm4SFPfY8WM8qkMARrAZWCf2m2kaAnNT1wEZvg727s_OfQ7qKx3MomTXmGNEviG517lU/s1600/bg.png") repeat scroll 0 0 transparent;border: 4px solid #EAEAEA;border-radius: 3px 3px 3px 3px;box-shadow: 3px 3px 0 #D6D6D6;margin-left: 48px;padding: 3px 10px;position: relative;}#comments .avatar-image-container img {border: 2px solid #FFFFFF !important;border-radius: 50px 50px 50px 50px;height: 57px;max-width: 57px;}.comments .avatar-image-container {border: 2px solid #FFFFFF;border-radius: 34px 34px 34px 34px;box-shadow: 1px 1px 3px #ABABAB;float: left;margin-left: -40px;max-height: 60px !important;overflow: hidden;width: 60px !important;}.comment-header {border-bottom: 2px solid #F1F1F1;}.comment-block:hover > .comment-header {-moz-transition: border-color 0.5s ease 0s;border-bottom: 2px solid #000000;}.comments .comment .comment-actions a {background: -moz-linear-gradient(center top , #EEEEEE 0%, #CCCCCC 100%) repeat scroll 0 0 transparent;border: 2px solid #E0E0E0;border-radius: 3px 3px 3px 3px;color: #000000;font: bold 12px arial;margin-right: 14px;padding: 4px 11px !important;text-decoration: none !important;text-shadow: 1px 1px 0 #FFFFFF;}