If more than one tax code is used on a transaction form, the tax rate for each will appear on individual lines. The total tax amount for each rate used will then be shown in the totals section at the bottom.
If only one tax code is used, that column is omitted, and only the total for the single rate used is shown in the totals section.
Being able to create custom themes would not help you achieve what you want in the latter situation, because the tax percentage is not an element of the table being displayed. That content is set by a deeper layer of code. Manager’s basic design philosophy is not to include unnecessary information.
There has, however, been some discussion about adding tax amounts to line items. We’ll have to wait to see what might happen.