The new Twitch theme (2019-09-26) is pretty bad imo. This makes it a little better.

style.css

body {
  font-family: Arial, sans-serif !important;
}

/*
 * (Optional) Remove annoying overlay
 */
.extensions-video-overlay-size-container,
.extensions-dock__layout {
  display: none !important;
}

/*
 * (Optional) Remove sad-ad about using an ad blocker.
 */
/** Only usable with CSS4.
.video-player__overlay:has([data-test-selector="sad-overlay"]) {
  display: none;
}
*/

/*
 * LIGHT THEME
 */

.side-nav-card__link {
  border-bottom: 1px solid #ccc;
}

/*
 * DARK THEME (overrides light theme)
 */

.tw-root--theme-dark {
  --color-background-input-focus: #222;
  --color-border-input-focus: #777;
}

.tw-root--theme-dark body,
.tw-root--theme-dark .channel-header,
.tw-root--theme-dark .channel-root__right-column {
  background: #242424;
}

.tw-root--theme-dark .chat-room {
  background: #242424;
}

.tw-root--theme-dark .side-nav-card__link {
  border-bottom: 1px solid #444;
}

/*
 * GLOBAL
 */

h1 {
  font-size: 3.5rem;
}

.navigation-link * {
  font-size: 14px !important;
}

.top-nav .tw-avatar__presence--offset {
  left: calc(86%);
  top: calc(86% - .3rem);
}

.side-nav-card__title p {
  font-size: inherit !important;
  line-height: 1.4 !important;
}

.channel-leaderboard-header__users .tw-strong {
  font-weight: normal !important;
}

.channel-leaderboard-header__users p {
  font-size: 12px;
}

Instructions:

1. Install a “User Styles Manager” extension in your browser of choice. I use Firefox and an extension called Stylus.
2. Copy the CSS code above.
3. Create a new user style for https://www.twitch.tv/ in your extension of choice and paste the CSS code. Stylus users can follow the instructions below.

Stylus extension instructions:

1. Open https://www.twitch.tv/
2. Click the “Stylus” icon in your browser toolbar.
3. Click “www.” in “Write style for: www.twitch.tv/this URL”
4. Paste the CSS code that you copied in step 2.
5. Save