Hi there!

Thanks for your query.

Did you mean all these thin borders? https://prnt.sc/hq4iI0MJO7mn

Then, please use the following code in your custom CSS to remove the border and place the featured image in the middle:

.article-list .article .article-intro-image img, .article-list .article .article-featured-video img, .article-list .article .article-featured-audio img, .article-list .article .article-feature-gallery img {
    margin: 0 auto;
}
.article-list .article, .article-list .article .article-intro-image, .article-list .article .article-featured-video, .article-list .article .article-featured-audio, .article-list .article .article-feature-gallery {
    border: none;
}

Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Hope this helps!

Best Regards