you could just convert the Slate theme to work with v8+. its not to hard.

I’ve copied the package over but it won’t install..
Says “Unknown package...Broken package (handle theme_slate)

I’m assuming the controller.php needs to be modified for it to install???

Actually I’ve modified controller.php to enable the package to be recognized and I can install it but it doesn’t add the theme.

I don’t have a page_theme.php file as mentioned here..
https://documentation.concrete5.org/developers/designing-for-concret...

Should I add one?

I would attack this the other way round. Use Elemental Cloner to create a starting point. Then replace the elemental css and less with css from the legacy slate theme.
That way you have all the scaffolding required by v8 themes from Elemental.

Obviously not that straight forward, as there are also custom blocks, block templates, css grid, bootstrap etc to resolve between the two. However, by working this way round you start with a theme that works and refine its design to match slate.

I see what you mean - structure of the package is quite different and will take a bit of sorting through

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