Make MyBB 1.8 minimum width fluid

AdminCP > Templates & Style tab > Themes, click Default, click Options button then Duplicate Theme (check box for Duplicate Templates). Name new theme: MyBB Master Style Fluid

For MyBB Master Style Fluid click Options button and select Set as Default.

Click options button for MyBB Master Style Fluid and choose Edit Theme then edit the following.

global.css

#container
Comment out CSS in .container like this:
/* min-width: 990px; */

.wrapper
Comment out CSS in .wrapper like this:
/* min-width: 970px; */

In Width field of .wrapper set this:
95%

Note: After the item is commented out it will be automatically deleted from style after browsing away from Edit Style Sheet: Simple Mode.

OR export theme from test forum (if you have one) and import to Your Forum and use the name: MyBB Master Style Fluid