I guess you want to address skins used on the admin and host pages and the skins that are used by the controls like the "Terms" and the "Login" page.
For these the align should be fixed to left and for the rest not, right?
There are 3 solutions for this.
1. Upgrade to DNN 5 ;-)
2. Use an exact copy of your skin with a forced test-align:left only as the Admin skin.
3. Use some conditional CSS, but you need some inline ASP.NET for that.
This is how I would do that
Wrap your skin in this div:
<div class="<%=IIf(Request.RawUrl.ToLower.Contains("/ctl/"), "AdminSkin", "NormalSkin") & " " & PortalSettings.ActiveTab.BreadCrumbs(0).TabName%>">
.....You skin.....
</div>
This will render:
A. Class "AdminSkin" for all pages that are not an Admin or Host page but do use the Admin skin (login, terms etc.)
B. The name of the root page
So with the following CSS you can cover all pages that use the Admin skin:
.AdminSkin .DNNAligncenter, .Admin .DNNAligncenter, .Host .DNNAligncenter{text-aling:left;}
.Admin for all admin pages, .Host for all Host pages and .AdminSkin for the rest
HTH