Hi Ansuman;
this is the configured https port of your system.
You can open the application also with https port or you can open theme designer with http port which is 8000 as you mentioned.
open theme designer change the protocol to http and port to 8000 it will work fine.
http://<host>:8000/sap/bc/theming/theme-designer
also for confirmation as mentioned in the above reply go to SMICM Goto->services will shown you the ports assinged for http and https.
Regards,
Sarbjeet Singh