Basic Theme – video thumbnail not responsive

I added few videos from my youtube channel using embed code. The video thumbnail is not responsive (does not fit best when viewed on mobile device)

I checked by adding below css code in menu Appearance > Customize > Custom codes in the block Custom Styles:

.ytp-cued-thumbnail-overlay-image {
background-size: contain;
-moz-background-size: contain;
-webkit-background-size: contain;
}

But it does not help. How can i make my video thumbnail responsive.

Thanks,
Raghav

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