JobRoller is an Application theme which runs off the WordPress platform (i.e. you must have WordPress installed in order to use JobRoller). Our themes are not encrypted, so if you have the coding knowledge to do so, you are free to customize as you wish.
If you haven't already done so, you can view the theme features on the
JobRoller overview page (which also includes link to the front end and admin demo).
I'm not sure what you mean by "Job Credits"? But you can enable the header banner and job listing banner from within the JobRoller admin dashboard. You may also find some WordPress plugins which will enable you to manage sidebar banner ads, or custom build ad spaces. There is no "upgrade" option for Job Seekers. User accounts can be managed from the general WordPress admin settings. For anything you wish to implement yourself, how "easy" this is, is really relative to your level of coding knowledge.