Updating Storybook base styles (#12862)

* Adding branding and updating styles for storybook docs

* Making code blocks light gray

.storybook/manager.js

@@ -0,0 +1,6 @@
+import { addons } from '@storybook/addons';
+import MetaMaskStorybookTheme from './metamask-storybook-theme';
+
+addons.setConfig({
+  theme: MetaMaskStorybookTheme,
+});

.storybook/metamask-storybook-theme.js

@@ -0,0 +1,10 @@
+// .storybook/YourTheme.js
+
+import { create } from '@storybook/theming';
+import logo from '../app/images/logo/metamask-logo-horizontal.svg';
+
+export default create({
+  base: 'light',
+  brandTitle: 'MetaMask Storybook',
+  brandImage: logo,
+});

.storybook/preview-head.html

@@ -0,0 +1,56 @@
+<link rel="preconnect" href="https://fonts.googleapis.com" />
+<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
+<link
+  href="https://fonts.googleapis.com/css2?family=Inconsolata&display=swap"
+  rel="stylesheet"
+/>
+
+<style>
+  * {
+    --blue-500: #037dd6;
+    --blue-400: #1098fc;
+    --gray-pre-bg: #f8f8f8;
+    --font-family-base: Euclid, Roboto, Helvetica, Arial, sans-serif;
+    --font-family-monospace: Inconsolata, monospace;
+    --font-size-code: 0.875rem;
+    --line-height-code: 19px;
+    --font-size-base: 1rem;
+    --line-height-base: 1.7;
+
+    font-family: var(--font-family-base) !important;
+  }
+
+  h1,
+  h2 {
+    font-weight: bold !important;
+  }
+
+  p,
+  li {
+    font-size: var(--font-size-base) !important;
+    line-height: var(--line-height-base) !important;
+  }
+
+  .docblock-source {
+    background: var(--gray-pre-bg) !important;
+  }
+
+  code {
+    font-family: var(--font-family-monospace) !important;
+    font-size: var(--font-size-code) !important;
+  }
+  code * {
+    font-family: var(--font-family-monospace) !important;
+    font-size: var(--font-size-code) !important;
+  }
+
+  a {
+    color: var(--blue-500) !important;
+  }
+
+  a:hover,
+  a:active,
+  a:focus {
+    color: var(--blue-400) !important;
+  }
+</style>