chore: Update itwinui-css to 1.0.0-dev.13

[gretanausedaite]

iTwinUI-react:

• Surface - update drop shadow to use variables
• Table - remove iui-icon from sort buttons
• colors - update soft backgrounds to use variables

Storybook:

• iTwin logo update (TODO: update logo in Overview page)
• itwintheme - update colors and add comments
• preview-head - update background to use background-backdrop
• preview - update background to use new variables and have fallback
• screenshots - updates images to use new colors and background
• stories:
  • InformationPanel - background update
  • SideNavigation - background update
  • Surface - elevation prop update, padding update

Website:

• _item.astro - background update
• LiveExample - update to use dev version of iTwinUI-react package
• index.astro - component example background update to better match new theme.

[mayank99]

Why do some images have grey bg but others white?

It looks really weird in some cases where the component blends in with the background.

[gretanausedaite]

Why do some images have grey bg but others white?

I did not find why backgrounds are white.