Custom invoice template

how can i change the theme on which i dont want to print code and want to have larger font for the rows on sale invoice?

Edit the code under Settings => Themes. If you do not have the skills, hire a programmer locally.

i managed to change it upto some extent but failed to exclude the item code column i.e

When creating an inventory item, the Item Code is optional.
If you leave the Item Code field blank it will not be shown on the sales invoice.
Item Name helps identify what item you are selling.
If you have no use of the item name, then when creating the inventory item enter the description of the item in the Item Name field and leave the Description field blank.

i have almost 2800 inventory items and all have item code how i can batch delete the item code olny. and will all reports and items work properly after deleting?

You can do a batch update of your inventory items. Check the guides section regarding this.