WordPress/wp-includes/blocks/rss/editor.css

.wp-block-rss li a>div{
  display:inline;
}

.wp-block-rss__placeholder-form>*{
  margin-bottom:8px;
}
@media (min-width:782px){
  .wp-block-rss__placeholder-form>*{
    margin-bottom:0;
  }
}
.wp-block-rss__placeholder-form .wp-block-rss__placeholder-input{
  flex:1;
  min-width:80%;
}