twitter_star.css

/* Reference: https://userstyles.org/styles/120402/twitter-s-fav-button-back */
/* The is the old default star, no animation */
.HeartAnimationContainer:before,.Icon--heart:before{content:"\f147";display:block;font-family:"rosettaicons";font-weight:normal;font-style:normal;text-align:center;-webkit-font-smoothing:antialiased;}
.ProfileTweet-action--favorite.withHeartIcon.is-animating .HeartAnimation,.HeartAnimation{display:none!important;}
.tweet-action[rel="favorite"]:hover,.tweet-detail-action[rel="favorite"]:hover,.tweet-action[rel="favorite"]:focus,.tweet-detail-action[rel="favorite"]:focus,.tweet-action[rel="favorite"]:active,.tweet-detail-action[rel="favorite"]:active,.tweet-action.is-selected[rel="favorite"],.Icon--favorited,.Icon--colorFavorited,.ProfileTweet-action--favorite .ProfileTweet-actionButton:hover *,.ProfileTweet-action--favorite .ProfileTweet-actionButton:focus,.ProfileTweet-action--favorite .ProfileTweet-actionCount:hover,.ProfileTweet-action--favorite .ProfileTweet-actionCount:focus,.tweet.favorited .tweet-actions .Icon--favorite,.tweet.favorited .ProfileTweet-actionList .Icon--favorite,.tweet.favorited .ProfileTweet-action--favorite .ProfileTweet-actionCount,.Icon--colorHeart,.Icon--circleActiveHeart,.Icon--heartBadge,.ProfileTweet-action--favorite.withHeartIcon .ProfileTweet-actionButton:hover,.ProfileTweet-action--favorite.withHeartIcon .ProfileTweet-actionButton:focus,.ProfileTweet-action--favorite.withHeartIcon .ProfileTweet-actionCount:hover,.ProfileTweet-action--favorite.withHeartIcon .ProfileTweet-actionCount:focus,.favorited .ProfileTweet-action--favorite .Icon--heart,.favorited .ProfileTweet-action--favorite.withHeartIcon .ProfileTweet-actionButtonUndo,.favorited .ProfileTweet-action--favorite .ProfileTweet-actionButtonUndo{color:#FFA200!important;}

.ProfileTweet-action--retweet .ProfileTweet-actionButton:hover,.ProfileTweet-action--retweet .ProfileTweet-actionButton:focus,.ProfileTweet-action--retweet .ProfileTweet-actionCount:hover,.ProfileTweet-action--retweet .ProfileTweet-actionCount:focus,.Icon--retweeted,.Icon--colorRetweeted,.ProfileTweet-action--retweet .ProfileTweet-actionButton:hover *,.ProfileTweet-action--retweet .ProfileTweet-actionButton:focus,.ProfileTweet-action--retweet .ProfileTweet-actionCount:hover,.ProfileTweet-action--retweet .ProfileTweet-actionCount:focus,.Icon--retweeted,.Icon--colorRetweeted,.tweet.retweeted .Icon--retweet,.tweet.retweeted .ProfileTweet-action--retweet .ProfileTweet-actionButtonUndo{color:#0F961A!important;}

.hearty .icon-favorite:before{content:"\f007"!important;}.Icon--heartBadge:before{content:"\f001"!important;}

.prf [data-type="favorites"]:hover *,.tweet-action[rel="favorite"]:hover,.tweet-detail-action[rel="favorite"]:hover,.tweet-action[rel="favorite"]:focus,.tweet-detail-action[rel="favorite"]:focus,.tweet-action[rel="favorite"]:active,.tweet-detail-action[rel="favorite"]:active,.tweet-action[rel="favorite"].is-selected,.is-favorite .icon-favorite-toggle,.hearty .icon-favorite-color{color:#FFA200!important;}

.tweet:before,.tweet:after,.ProfileTweet-header:before{display:inline-block;vertical-align:text-top;background-image:url(https://abs.twimg.com/a/1431997920/img/t1/twitter_web_sprite_icons.png);
background-repeat:no-repeat;position:absolute;top:0;right:0;visibility:hidden;width:24px;height:24px;}

.favorited:before,.favorited:after,.is-favorited .ProfileTweet-header:before{content:" ";background-position:-30px -450px;visibility:visible;}

.retweeted:before,.retweeted:after{content:" ";background-position:0 -450px;visibility:visible;}
.favorited.retweeted:before,.favorited.retweeted:after,.is-retweeted .ProfileTweet-header:before,.is-retweeted.is-favorited .ProfileTweet-header:before{content:" ";background-position:-60px -450px;visibility:visible;}


/* Belows are imitated star, with animation */
/*
.HeartAnimation {
    background: url("https://pbs.twimg.com/media/CS9U_wmUsAAzuUz.png:large");
    background-size: 2900%;
    background-position: left;
}

.ProfileTweet-action--favorite .ProfileTweet-actionButton:hover .HeartAnimation, .ProfileTweet-action--favorite .ProfileTweet-actionButton:focus .HeartAnimation, .favorited .ProfileTweet-action--favorite .HeartAnimation {
    background-position: right;
}

.Icon--heartBadge {
    color: #ffac33;
}

.Icon--heartBadge:before {
    content: "";
}
*/

adopted a CSS found on user-style by default (the old star favour). If you like the animated one, comment out the upper section and uncomment the lower part