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/theme/empty.css

@@ -0,0 +1,21 @@
+/* Empty states (or Blank slates) */
+.empty {
+    background: var(--body-color-contrast);
+    border-radius: var(--border-radius);
+    color: var(--secondary-text-color);
+    text-align: center;
+    padding: var(--unit-16) var(--unit-8);
+
+    .empty-icon {
+        margin-bottom: var(--layout-spacing-lg);
+    }
+
+    .empty-title,
+    .empty-subtitle {
+        margin: var(--layout-spacing) auto;
+    }
+
+    .empty-action {
+        margin-top: var(--layout-spacing-lg);
+    }
+}