Theme improvements (#822)

* start converting

* small fixes

* reorganize theme files

* cleanup search bar

* increase spacing

* small tweaks

* fix select styles in Chrome

* cleanup menus

* improve button icons

* restore badges

* remove unused classes

* restore some overrides

* restore bookmark form

* add summary outline

* avoid layout shifts

* restore bookmark details

* increase border radius for modals

* improve details modal

* restore reader mode

* restore settings

* cleanup variables

* start with dark theme

* more dark theme...

* more light theme...

* more dark theme...

* add postcss build

* remove sass processor

* update docker build

* fix alt color

* remove endless symbol

* fix tests

* update assets

* remove sass files

* fix docker build

* cleanup spacing

* improve theme

* update test scripts

* update CI workflow

* fix test

bookmarks/styles/settings.css

@@ -0,0 +1,26 @@
+.settings-page {
+  section.content-area {
+    margin-bottom: var(--unit-10);
+
+    h2 {
+      margin-bottom: var(--unit-3);
+    }
+  }
+
+  textarea.monospace {
+    font-family: monospace;
+    box-sizing: border-box;
+  }
+
+  .input-group > input[type=submit] {
+    height: auto;
+  }
+
+  section.about table {
+    max-width: 400px;
+  }
+
+  & .form-group {
+    margin-bottom: var(--unit-4);
+  }
+}