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/base.css

@@ -0,0 +1,61 @@
+/* Base */
+*,
+*::before,
+*::after {
+    box-sizing: inherit;
+}
+
+html {
+    box-sizing: border-box;
+    font-size: var(--html-font-size);
+    line-height: var(--html-line-height);
+    -webkit-tap-highlight-color: transparent;
+    scrollbar-gutter: stable;
+}
+
+/* Reserve space for vert. scrollbar to avoid layout shifting when scrollbars are added */
+html {
+    scrollbar-gutter: stable;
+}
+
+@media (pointer: coarse) {
+    html {
+        scrollbar-gutter: initial;
+    }
+}
+
+body {
+    background: var(--body-color);
+    color: var(--text-color);
+    font-family: var(--body-font-family);
+    font-size: var(--font-size);
+    overflow-x: hidden;
+    text-rendering: optimizeLegibility;
+}
+
+a {
+    color: var(--link-color);
+    outline: none;
+    text-decoration: none;
+}
+
+a:focus-visible {
+    outline: var(--focus-outline);
+    outline-offset: var(--focus-outline-offset);
+}
+
+a:focus,
+a:hover,
+a:active,
+a.active {
+    text-decoration: underline;
+}
+
+summary {
+    cursor: pointer;
+}
+
+summary:focus-visible {
+    outline: var(--focus-outline);
+    outline-offset: var(--focus-outline-offset);
+}