Adopt docs color palette to lens styleguide + makes dark theme default (#2804)

Signed-off-by: Mario Sarcher <mario@sarcher.de>

docs/stylesheets/extra.css

@@ -5,13 +5,38 @@
 
 :root > * {
   /* Footer */
-  --md-footer-bg-color:         #3d90ce;
+  --md-footer-bg-color:         #e8e8e8;
+  --md-footer-bg-color--dark:   #cccdcf;
+  --md-footer-fg-color:         #3d90ce;
+  --md-footer-fg-color--light:  #fff;
+
+  /* Background */
+  --md-default-bg-color:        #f1f1f1;
+}
+
+[data-md-color-scheme="slate"] {
+  /* Footer */
+  --md-footer-bg-color:         #2e3136;
+  --md-footer-bg-color--dark:   #262b2f;
+  --md-footer-fg-color:         #3d90ce;
+  --md-footer-fg-color--light:  #fff;
+
+  /* Background */
+  --md-default-bg-color:        #1e2124;
 }
 
 .md-version__list {
   overflow: auto;
 }
 
+.md-header-nav__title {
+  margin-left: 0;
+}
+
+.md-header__title {
+  margin-left: 0;
+}
+
 ul.video-list {
   counter-reset: section;
   list-style: none;