is that possible, can I use if I downgrade the luci version?, I want to use apollo-ng/darkmatter theme
You need to ask the author.
That third-party theme has never been officially supported.
You can't downgrade the actual underlying Luci version, as it is tightly tied to the corresponding OpenWrt version.
I am asking this
Neither of those things are supported, nor likely to work - if you want to pursue this, you're quite literally on your own. It would be better if you'd take the opposite approach, fixing the theme to work with current luci, while that will still require quite some effort, it should be considerably more likely to be possible than your approaches (while the bootstrap and openwrt themes needed constant maintenance to keep working, the changes haven't been that fundamental over the last decade either).