This issue occur when you have not applied the patches, please follow the below instruction
- If you use magento 1.9.1.x, so please apply the content of patch1.9.1.x to magento root on your server
- If you use magento 1.9.2.x, so please apply the content of patch1.9.1.x and 1.9.2.x to magento root on your server ( 1.9.1.x -> 1.9.2.x )
- If you use magento 1.9.3.x, so please apply the content of patch1.9.1.x, 1.9.2.x and 1.9.3.x to magento root on your server ( 1.9.1.x -> 1.9.2.x -> 1.9.3.x ).
Also you should disable cache and compilation while using the frontend builder.
If you want to add more css code, the best way is you should create an other custom file and add your css code to it, I will guide 1) Create a file mycustom.css in app\design\frontend\Mgs\claue\web\css\ folder
2) Declare it in \app\design\frontend\Mgs\mgsblank\Magento_Theme\layout\default.xml as the way we declared other css file
3) You can add your css to mycustom.css file but you must deploy again to your change reflect to the frontend. Note to remove the content of var and pub/static/frontend/ folder before deploying
4) If you don't want to deploy again each add css to file, you can add your css to mycustom.css file in pub/ folder directly but note after edting it, you must copy the changes to mycsutom.css file in app/design/ folder