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/responsive.css

@@ -0,0 +1,116 @@
+.show-sm,
+.show-md {
+  display: none !important;
+}
+
+.width-25 {
+  width: 25%;
+}
+
+.width-50 {
+  width: 50%;
+}
+
+.width-75 {
+  width: 75%;
+}
+
+.width-100 {
+  width: 100%;
+}
+
+.container {
+  margin-left: auto;
+  margin-right: auto;
+  width: 100%;
+  max-width: var(--size-lg);
+}
+
+.grid {
+  --grid-columns: 3;
+  display: grid;
+  grid-template-columns: repeat(var(--grid-columns), 1fr);
+  grid-gap: var(--unit-4);
+}
+
+.grid > * {
+  min-width: 0;
+}
+
+.columns-2 {
+  --grid-columns: 2;
+}
+
+.gap-0 {
+  gap: 0;
+}
+
+.col-1 {
+  grid-column: span min(1, var(--grid-columns));
+}
+
+.col-2 {
+  grid-column: span min(2, var(--grid-columns));
+}
+
+.col-3 {
+  grid-column: span min(3, var(--grid-columns));
+}
+
+@media (max-width: 840px) {
+  .hide-md {
+    display: none !important;
+  }
+  .show-md {
+    display: block !important;
+  }
+
+  .width-md-25 {
+    width: 25%;
+  }
+  .width-md-50 {
+    width: 50%;
+  }
+  .width-md-75 {
+    width: 75%;
+  }
+  .width-md-100 {
+    width: 100%;
+  }
+
+  .columns-md-1 {
+    --grid-columns: 1;
+  }
+  .columns-md-2 {
+    --grid-columns: 2;
+  }
+}
+
+@media (max-width: 600px) {
+  .hide-sm {
+    display: none !important;
+  }
+  .show-sm {
+    display: block !important;
+  }
+
+  .width-sm-25 {
+    width: 25%;
+  }
+  .width-sm-50 {
+    width: 50%;
+  }
+  .width-sm-75 {
+    width: 75%;
+  }
+  .width-sm-100 {
+    width: 100%;
+  }
+
+  .columns-sm-1 {
+    --grid-columns: 1;
+  }
+  .columns-sm-2 {
+    --grid-columns: 2;
+  }
+}