Block Editor (Gutenberg) buttons are broken

[Zodiac1978]

This is because of not using text-decoration: underline, but border-bottom for underlining the (linked) buttons here:

.page-content a:hover, .page-content a:focus, .entry-content a:hover, .entry-content a:focus, .entry-summary a:hover, .entry-summary a:focus, .comment-content a:hover, .comment-content a:focus {
    border-bottom: 3px solid #c3c3c3;
    box-shadow: inset 0 -3px 0 #c3c3c3;
}