Import Bootstrap after importing our variable overrides (#2066)

This PR restores the effects of the variables defined in
[variables/_bootstrap.scss](https://github.com/pydata/pydata-sphinx-theme/blob/main/src/pydata_sphinx_theme/assets/styles/variables/_bootstrap.scss)

This fixes #2022 and
also fixes another issue that I had noticed when clicking links in the
"More" dropdown. Screenshots:

- Fixed: 

![](https://github.com/user-attachments/assets/060fa3fe-2ce8-4311-b127-7821ddbbdadc)
- Broken: 

![](https://github.com/user-attachments/assets/f1de59b9-e6a6-485f-8e83-22a3548de94f)

src/pydata_sphinx_theme/assets/styles/pydata-sphinx-theme.scss

@@ -1,4 +1,9 @@
-// Import all of Bootstrap scss
+// IMPORTANT: our Bootstrap variable overrides must be imported before the
+// Bootstrap files that reference those variables; otherwise our overrides will
+// have no effect.
+@import "variables/bootstrap";
+
+// Import all of Bootstrap scss.
 @import "~bootstrap/scss/bootstrap";
 
 // Import FontAwesome from _node_modules
@@ -8,7 +13,6 @@
 @import "~@fortawesome/fontawesome-free/scss/brands";
 
 // Variables
-@import "variables/bootstrap";
 @import "variables/layout";
 @import "variables/fonts";
 @import "variables/icons";