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/animations.css

@@ -0,0 +1,38 @@
+/* Animations */
+@keyframes loading {
+    0% {
+        transform: rotate(0deg);
+    }
+    100% {
+        transform: rotate(360deg);
+    }
+}
+
+@keyframes slide-down {
+    0% {
+        opacity: 0;
+        transform: translateY(calc(-1 * var(--unit-8)));
+    }
+    100% {
+        opacity: 1;
+        transform: translateY(0);
+    }
+}
+
+@keyframes fade-in {
+    0% {
+        opacity: 0;
+    }
+    100% {
+        opacity: 1;
+    }
+}
+
+@keyframes fade-out {
+    0% {
+        opacity: 1;
+    }
+    100% {
+        opacity: 0;
+    }
+}