Themes aren’t working after 1.2 to 1.5 upgrade

Do you have any old .css files left over in your WP directory? Sometimes the browsers use any style they can find. I had some problems getting my theme to display and when I deleted the old files, it fixed everything.

No I don’t. I guess the real question is what happens when switching themes? Does it copy the index file to your root directory or what?

Give this a good read:
https://codex.www.ads-software.com/Using_Themes

I’ve been through that but it’s not really answering my question. Right now I have index files in three locations. https://www.mainsite.com/index.php https://www.mainsite.com/wp/index.php and https://www.mainsite.com/wp/wp-content/themes/maintheme/index.php. If you go to https://www.mainsite.com it’s obviously going to use the index.php file there. I didn’t read anything in any of the documentation that explains what to do with your existing index and css files.

OK, I think I just figured it out. I don’t know why that was so confusing.

mainsite.com/wp/index.php is the index which came with wp
mainsite.com/index.php is as above, however change the line in there to require(‘./wp/wp-blog-header.php’);
the index in the theme should be whatever index came with that theme. WP doesnt move any files, its all in the database so its dynamic. Try using the default or classic themes (unaltered by yourself) to ensure youve not broken anything in your own theme.

what to do with your existing index and css files.

See: https://www.tamba2.org.uk/wordpress/upgrade/