How to increase width of main container and decrease width of sidbar widget

@robin69, you can do that by modifying the width percentage on the containers with custom CSS in the Appearance > Customize > Custom CSS section.

For eg:

@media screen and (min-width: 60em) {
    .has-sidebar .site-main {
        max-width: 70%;
        float: left;
    }
    .has-sidebar .primary-sidebar {
        max-width: 30%;
        float: right;
    }
}

Adjust the percentage numbers on the max-width rule as you see fit. Default is 66-33.

This setting is not working for my blog. Plz, help me with an alternative way.

@rahulrainbow, I tried applying it again on the demo setup and its working fine for me. Did you follow the instructions and paste it correctly in the customizer? Also, did you change the width % numbers to what you want?

Does it work when you apply !important to the rule?

E.g: max-width: 70% !important;