I have tried using javascript
document.getElementById('fieldname').style.textAlign="right";
as well as changing CRMWeb\_forms\controls.css direction of INPUT.ms-crm-Number field's 'direction' attribute from 'ltr' to 'rtl' and changing text-align
attribute to 'right'
but nothing seems to work. I just want to change alignment for few of the number(decimal) fields not default css of all the number(decimal) fields.