To make a fixed version of your style, hat you could do is this.
ACP --> Style & Templates --> Style Manager --> Download / Upload Styles
In that screen, in the
Download Block, choose your style, it the field,
Title, add the word,
Fixed to the end. For the select option,
Options, chose,
Get customizations made in this style and all parent styles. Then click on
Download.
While still on that screen, in the block
Import Style XML File, click on
Browse, find your style you just downloaded, In the field,
Parent Style, choose the style name you just downloaded / copied from, then click on
Import.
Now you will have a new style on your site called XXXX Fixed. So go to the,
ACP --> Style & Templates --> Style Manager --> Find the style you just imported, and select
Style Variable Editor.
In that screen scroll down till you see the group called,
Global. In that group, find the stylevar,
doc_margin, and set the
Use the 'top' value for all sides to
No, and the
Left & Right fields to
auto, then click on
Save. In that same group, edit the stylevar,
doc_maxWidth, and change
Units to
px and change
Size to
980, then click on Save. Yo can always change the
980 later to a different size it you don't like the fit.
So now you have a copy of your style that is selectable by your members, in the style chooser dropdown, and is a copy of your fluid style, and best of all, the way I showed you to do it is that any template changes you make in the style you copied from, will automatically show in this style.