Hides some YouTube annoyances, and add some visual tweaking.

pimp-yt-player.css

/* Rules for Stylus
 * https://addons.mozilla.org/en-US/firefox/addon/styl-us/
 */

/* Hide the annoying (i) button on the top-right corner */
.ytp-chrome-top-buttons {
  display: none;
}

/* Display the controls and title only on hover */
.ytp-chrome-top,
.ytp-chrome-bottom {
  opacity: 0;
  background-color: #30303080;
  position: absolute;
  width: 100% !important;
  left: 0 !important;
}
.ytp-chrome-top:hover,
.ytp-chrome-bottom:hover {
  opacity: 1;
}

/* Position the background for the title and controls */
.ytp-chrome-top {
  background-position: top;
}
.ytp-chrome-bottom {
  background-position: bottom;
}

/* Change the padding of the title */
.ytp-title-text {
  padding: 10px 20px 5px !important;
}

/* Display the title on hover, without being fullscreen */
.ytp-hide-info-bar .ytp-chrome-top {
  height: auto;
}
.ytp-hide-info-bar .ytp-chrome-top .ytp-title {
  display: flex;
}

/* Hide the dark gradient on the title and controls */
.ytp-gradient-top,
.ytp-gradient-bottom {
  display: none;
}

/* Limit the width of the title on the description area */
.watch-title-container {
  max-width: 370px;
}

/* Hide the 'More Videos' button and suggestions on pause */
button.ytp-button.ytp-expand,
.ytp-pause-overlay {
  display: none !important;
}

/* Hide suggestions at the end of the video when in fullscreen */
.ytp-ce-top-left-quad.ytp-ce-size-1280 {
  display: none;
}