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/layout.css

@@ -0,0 +1,39 @@
+/* Main layout */
+body {
+    margin: 20px 10px;
+
+    @media (min-width: 600px) {
+        /* Horizontal offset accounts for checkboxes that show up in bulk edit mode */
+        margin: 20px 32px;
+    }
+}
+
+header {
+    margin-bottom: var(--unit-9);
+
+    .logo {
+        width: 28px;
+        height: 28px;
+    }
+
+    a:hover {
+        text-decoration: none;
+    }
+
+    h1 {
+        margin: 0 0 0 var(--unit-3);
+        font-size: var(--font-size-lg);
+    }
+}
+
+header .toasts {
+    margin-bottom: 20px;
+
+    .toast {
+        margin-bottom: 0.4rem;
+    }
+
+    .toast a.btn-clear:visited {
+        color: currentColor;
+    }
+}