This is a user style that I managed to come up with in about 15 minutes, based on a cached copy of a question with old styles (since then gone from cache).
button:is(.js-vote-down-btn, .js-vote-up-btn, .js-upvote-btn, .js-downvote-btn) { border: none !important; border-radius: 0 !important; background: none !important; padding: 0 !important; } button:is(.js-vote-down-btn, .js-vote-up-btn, .js-upvote-btn, .js-downvote-btn):hover { background: none !important; } button:is(.js-vote-down-btn, .js-vote-up-btn, .js-upvote-btn, .js-downvote-btn) { color: inherit !important; } button:is(.js-vote-down-btn, .js-vote-up-btn, .js-upvote-btn, .js-downvote-btn)[aria-pressed="true"] { color: var(--theme-primary) !important; } button:is(.js-vote-down-btn, .js-vote-up-btn, .js-upvote-btn, .js-downvote-btn) > svg { width: 36px; height: 36px; }
This only resets the button appearance; to also revert the new style of the vote counts, you need to also add:
.js-vote-count { color: var(--black-500) !important; font-size: var(--fs-title) !important; padding: 0 !important; font-weight: normal !important; }
Install it with your favourite user style manager; as of this writing, I use Stylus. As of version 120, Firefox for Android has finally re-enabled most extensions and Stylus can be installed there.