Is it possible modify the css of form login page? The version of thingworx is 8.2.
Yes. You can do inline CSS. Keep in mind, though, that any changes you make to the login page should be included in your source control. Any time you update, your changes will be overwritten and you will need to restore your updates.
I'm not going to get in to the programming concepts. For that, google is your friend. Search on inline CSS in JSP. That should get you what you need.
The JSP page you need to modify will be replaced automatically whenever you install a new version of TWX. You will want to ensure your, now custom, JSP page is backed up somewhere (preferably in a source control system) so that you can easily restore your custom JSP page after doing a TWX update.