feat(theme): persist user theme preferences (#2756)

- Migrate theme persistence from `sessionStorage` to `localStorage`
- Rename theme HTML attribute to `data-bs-theme` for Bootstrap compatibility
- Trim and compile CSS according to the chosen theme mode

_sass/abstracts/_mixins.scss

@@ -1,3 +1,9 @@
+@mixin color-scheme($mode) {
+  @media (prefers-color-scheme: #{$mode}) {
+    @content;
+  }
+}
+
 @mixin text-ellipsis {
   overflow: hidden;
   text-overflow: ellipsis;

_sass/abstracts/_variables.scss

@@ -28,3 +28,9 @@ $code-icon-width: 1.75rem !default;
 
 $font-family-base: 'Source Sans Pro', 'Microsoft Yahei', sans-serif !default;
 $font-family-heading: Lato, 'Microsoft Yahei', sans-serif !default;
+
+/* Theme mode settings */
+
+$theme-attr: 'data-bs-theme'; /* the attribute used to indicate the resolved theme */
+$theme-options: light, dark;
+$theme: null !default; /* set by Jekyll site configuration */