One of the baseline principles of web accessibility is that all functionality of a site should be available via keyboard navigation alone. It's an important baseline to accommodate those with muscular difficulties or those who are blind (and would rely on a screen reader), who would be unable to use a mouse.
Users reliant on a keyboard (including blind users) cannot upvote or downvote posts, or upvote comments. The buttons cannot be reached via the tab key, and can only be reached via clicking them with the cursor.
As with the comment flag button being unreachable, this occurs because it is an anchor element that isn't a link and which has no tabindex set. Fixing it would involve making it either a
<button type="button"> or giving it the attributes
<a tabindex="0" role="button">. ARIA guidelines would suggest use the button element, but if it must stay an anchor element, you want to give it the button role because it isn't a link or anchor, it's a button.
Thankfully the accept checkmark is keyboard accessible and exists in tab order (because it has a href attribute), but they cannot give their feedback via voting.
This bug existed in the old-style themes and persists in the new themes being rolled out.