bug #1756 [Site] Fix input range colors (Kocal)

This PR was merged into the 2.x branch.

Discussion
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[Site] Fix input range colors

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

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Following #1743 (comment), removing the custom CSS and configuring Bootstrap variables made the `.form-range` displays correctly in both Chrome and Firefox:

<img width="1528" alt="image" src="https://github.com/symfony/ux/assets/2103975/0a45fa49-5ff4-4da2-869d-16e875eef20c">

<img width="1433" alt="image" src="https://github.com/symfony/ux/assets/2103975/f2065757-3e27-45e1-9e4f-e471603e9118">

Commits
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6879b28 [Site] Fix input range colors

Author: kbond

ux.symfony.com/assets/styles/_variables.scss

@@ -18,3 +18,6 @@ $primary: #222 !default;
 $size-unit: calc(8px + 1.5625vw);
 
 $font-family-monospace: var(--font-family-code);
+
+$form-range-track-bg: var(--bs-body-bg);
+$form-range-thumb-bg: var(--bs-secondary-color);

ux.symfony.com/assets/styles/vendor/_bootstrap.scss

@@ -37,12 +37,3 @@
   }
 
 }
-
-
-.form-range::-moz-range-track {
-  background: var(--bs-secondary-bg-subtle);
-}
-
-.form-range::-moz-range-thumb {
-  background: var(--bs-secondary-color);
-}