CSS indented with 4 spaces should be 2 spaces

Problem/Motivation

#2815831: Move Off-canvas related CSS from drupal.outside_in library to drupal.off_canvas → recently went in and added two CSS files with incorrect indents.
While searching, found a few more

$ ag -l -G="*.css" "^\S.*{\n    "
core/modules/outside_in/css/offcanvas.css
core/modules/outside_in/css/offcanvas.motion.css
core/modules/system/css/system.admin.css
core/modules/views_ui/css/views_ui.admin.theme.css
core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css
core/themes/bartik/color/preview.css
core/themes/stable/css/system/system.admin.css
core/themes/stable/css/views_ui/views_ui.admin.theme.css

I left core/tests/Drupal/Tests/Core/Asset/css_test_files/quotes.css alone since it is for testing whitespace and CSS aggregation.

Proposed resolution

Fix the remaining files.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A