It sounds very much like your colour overlay file is not getting loaded.
if you go to the forum admin > themes panel do you have an overlay selected from the list and if not can you select one and update it and see what happens next? Actually try updating no matter what.

it is possible for your host to restrict css files from having php in theme with certain mime settings…

can you try the css only theme and see if it works?  if so, might contact your host too..

Thanks YS –  that seems to have done the trick when I tested it – I have all the themes loaded but of course only one at a time is used!  

I will keep an eye on the logs.

Mr Papa said
can you try the css only theme and see if it works?  if so, might contact your host too..

Which is the css only theme?  Although as I am my own host I don’t have problems in that area!

(I have 19 themes loaded from the standard pack.)

good news! thanks for the update…

lol, the one title CSS Only… after download you only get the Default theme… you can get the other three themes we have (CSS Only, Stacked and iForum) on our downloads page…

Well it appeared to work but this morning there more slews of undefined variables in the error log.

Some users who login to the site then when they go to the forum, the login button is still there so they try to login again and get sent back to the main page.

The plugins I have installed (on WP 3.4.1) are:

bad-behavior, captcha, contact-form-plugin, fv-gravatar-cache, google-analytics-dashboard, invisible captcha, p3-profiler, really-simple-facebook-twitter-share-buttons, search-meter, simple-press, subscribe2, theme-my-login, tinymcecomments, top-commentators-widget, twitter-tools, uk-cookie-consent, uk-time, ultimate-tag-cloud-widget, wordpress-importer, wordpress-seo, wordpress-simple-paypal-shopping-cart, wp-maintenance-mode, wp-monalisa

All of these are up-to-date

The only one that affects the login is theme-my-login which does interfere with some plugins (notable s2member) but has some very nice features.

Any clues?

Following a suggestion on the WordPress forum I found an old copy of Theme My Login 6.1.4 and downgraded it and it works now.  I couldn’t downgrade Simple:Press as there had been a database modification.  The WP forum comments that it is some strange behaviour on SP upgrade and not the ‘fault’ of TML.  Is this true?

not that we are aware of… to date, we dont know of any issue with simple press and theme my login… have quite a fair number of folks using both…

but we will have to check out that thread in more detail… but bluantinoo had worked it out, I thought…

I don’t think the issue was resolved because it was solved but because it was not to do with TML which is the subject of that forum.  But it does imply s/problem-with-post-edit-buttone was talking to you about it…

when I have some spare time I will try TML out. It has been a long time since I last looked at it.