bug #1617 Fix styles (javiereguiluz)

This PR was merged into the main branch.

Discussion
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Fix styles

Without this change, I don't see any styles on the app and the server shows this:

```
[Application] Nov  4 14:19:23 |CRITICA| REQUES Uncaught PHP Exception
Symfony\Component\AssetMapper\Exception\RuntimeException:
"Unable to find asset "./bootswatch/_variables" referenced in "demo/assets/styles/_variables.scss".
The file "demo/assets/styles/bootswatch/_variables" does not exist."
at CssAssetUrlCompiler.php line 111
```

Commits
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28c46a6 Fix styles

Author: javiereguiluz

assets/styles/_variables.scss

@@ -23,4 +23,4 @@ $btn-padding-x: 10px;
 $btn-padding-y-sm: 9px;
 $btn-padding-x-sm: 6px;
 
-@import "./bootswatch/variables";
+@import "./bootswatch/_variables.scss";