Hi Rohanbanza,
It looks like stubborn Theme. ??
If theme author or support cannot help, that is tough one.
Reinstalling WordPress and your Theme can help. But you have to have your site backed up.
Do you have installed just your Theme or there is some other theme in The Dashboeard/appearance/themes?
You should have installed one more theme at least. Like Elevenfourteen or so. So theme which comes with wordpress.
Try to disable all you plugins except your simple custome css plugin and then try put in some css to it.
If it works them turn the plugins one by one and test if you can put any css to make a change.
It this is not working then turn off simple custome css and put some code to the main style.css file by going to the Dashboard/appearance/editor and on the right side choose your theme (top right) and chose style.css file from the list under. It wil open the file in the editor then Put some code to the bottom of the file, save a take a look.
One realy stupid, but for sure. Do you hit save in simple custom css when you put some css to the file?
Sory for that question, but I had one person that forget to safe and I could not help. ??
Cheers
TR