Why? That could potentially be done but would require a significant amount
of work. I can't imagine what you would gain.

Our organization does not like the out-of-the box colors and
structure of SPS. They would like to customize every WSS site with
their style and design. Now if the portal  is not customized then
there will be no continuity between the portal and the sites.
Although they would not gain anything from doing it, it adds
continuity to the rest of the websites.

I am kinda doing this now. I am simply editing the style sheets of
the portal  and the WSS sites so they look the same and match our brand
when a new WSS site is created.

CSS is certainly another option.

You should be able to theme your Portal in FrontPage 2003. However, you will
have to theme each new Area added when it is added.

portal cannot be themed in FrontPage 2003 u need to change the
css files. No other way.

You cannot apply a theme in SharePoint portal  Server.
Gotta love that, sorry for the mis-information! Crack open the CSS manual.
Remember, "changes made to OWS.CSS may be lost when [SharePoint] is updated,
such as through installation of service packs or patches. An alternate .css
file can be specified in the ONET.XML file of a site definition by using the
AlternateCSS attribute of the Project element.

The info on what to change where is available in this page. I managed
to make some changes looking at this.

msdn.microsoft.com/.../...zingSharePointSites3.asp

Also,

To keep things simple here (do not know whether we achieved that!) for
PORTAL style we did the following

1) kept the MS default css in place
2) copied contents of the above files each into a new
ourstyle.css file in sequence that SPS "loads them"
3) made changes in ourstyle.css to suit our color scheme and
also some images (replacing existing) with appropriate commenting
4) created ourstyle image directories and placed outstyle.css
referenced images in there

We did play around with the ows.css file. However, they were not too
impressed with just being able to change colors and existing styles
to match company colors and logos. They are talking about a complete
revamp in design structure of the portal  page without loosing any of
the already built-in functionality (search, areas etc). In short
words here is the link to a web template for a WSS site.

www.sharepointcustomization.com/.../default.aspx

So the question is can the portal be customized to look exactly like
this? using the css alone will not produce this result. Any
suggestions on how other organizations who wants to customize the
portal maybe taking the approach?

Also,

The link provided is a demonstration of how to customise a WSS site. You can go some way to getting the portal to look like that, but it requires editing of the various area templates, editing of the home page with FrontPage and you will still find some things that you can't easily change, like the top navigation bar, because they are web parts and thus tricky to alter the look and feel beyond what Microsoft permit.

There are several KB articles on msdn.microsoft.com about customisation of SPS and a quick search on there should give you the three-part starting set.

There is a 'nifty' way to get
continuity and apply themes on portal  site to match your wss  sites.
However the change will not be global and would have to be applied on
every site (areas) individually. Hence the term 'nifty'.

Type this: http://[your SPS address]/_layouts/1033/themeweb.aspx

You can apply your customized theme (assuming you have already
customized a theme and installed it based on your organizations
specs) on the portal. You will need to do this for every areas and
webpages pertaining to the portal. (not a smart move if you have lots
of areas and subareas) There is a way to apply it on the public view
of my sites as well. So now, the portal, wss sites, and public views
of mysites have continuity.

you can actually use this technique generally. Most
Windows SharePoint Services ASPX pages  in [sitename]/_layouts/1033 (or
whatever LCID you installed SharePoint into) can be used on a SharePoint
Portal Server even if SharePoint Portal Server doesn't expose them in the
Admin pages.

Unfortunately, this action only applies to "Home" area. Any other
area will produce "Access denied" error.

However when I tried it, we
only had Home as the area in our organization. If anyone finds a
solution to this let me know too. The closest I got was by creating a
subweb using the parent's template. However you then are restricted
to that parent template and moreover subwebs created from parent
template are not actual urls but catalog ids in SQL that point to the
same page. Which means that any change you do there affects the
parent page as well.