Custom themes is not a solution. Legal requirements are not meant to be solved by custom themes.
As far as I can see, Manager complies with UAE requirements.
Tax invoice title should be mentioned on all invoices issues
This can be achieved by having Custom Title option checked on invoice.
Tax Registration Number (TRN) to be displayed
This can be mentioned as part of your business address under Settings → Business Details
Price displayed should be inclusive of VAT
This can be done by checking the box Amounts tax inclusive on invoice.
Amount of VAT charged
Manager shows VAT charged amount on each invoice.
The only thing Manager doesn’t show (if amounts are tax-inclusive) is invoice total excluding tax. I mean if multiple people from UAE claim that this is issue, then of course it will be addressed promptly.