Press cmd + s / ctrl + s to save your unsaved changes.
Press cmd + z / ctrl + z to undo a change.
Press cmd + y / ctrl + y or cmd + shift + z / ctrl + shift + z to redo a change.
When editing a number value (such as Font Size or gutter Height), use the Up and Down arrows to increase and decrease the value:
Hold shift in order to increase and decrease by 10 at a time:
You can find these keyboard shortcuts in the editor View-tab: