Here's a list of CSS modifications that used to be common, but no longer are necessary as there are styling settings and other functions that are easier to use and maintain.
CSS Modification
Area
How to do instead
Add sheet background color and image.
Sheet
Use the sheet styling property.
Remove the sheet title.
Sheet
Use the app UI settings.
Remove the toolbar.
Sheet
Use the app UI settings.
Hide the buttons in the pivot table.
Pivot
Use the new pivot table.
Change the null color in the table.
Table
Use the null value styling settings in the new tables.
Pajama stripes in the table
Table
Use the zebra stripe styling setting in the new table.
Chart background color and image
Chart
Use the chart styling settings.
Chart border and shadow, color, radius and size.
Chart
Use the chart styling settings.
Change font family, weight, color and size.
Chart
Use the chart styling settings.
Change selection bar colors.
App
Use theme styling properties.
Hide popup buttons.
Chart
Turn off in app settings or per chart in chart settings.
