feat: add default CSS variables for consistent styling

- Introduced a new _defaults.scss file to define CSS variables for slider and range track styles.
- Updated index.scss to include the new defaults for improved styling consistency across components.

Author: atyrode

src/frontend/index.scss

@@ -1,6 +1,7 @@
 @import 'src/css/_colors.scss';
 @import 'src/css/_excalidraw-overrides.scss';
 @import 'src/css/_fonts.scss';
+@import 'src/css/_defaults.scss';
 
 /* Makes excalidraw fill the entire screen */
 #root {

src/frontend/src/css/_defaults.scss

@@ -0,0 +1,6 @@
+:root {
+    --slider-thumb-size: 16px;
+    --range-track-filled: #{$accent};
+    --range-track-unfilled: #{$grey-800};
+    --range-thumb-color: #{$accent-dark};
+  }