My site is currently on C5.6.4.0. In a separate directory I have installed C5.8.x.x and imported and selected the theme. However when I try to edit the homepage I don’t get any editable areas. Just blank. I have emptied the cache but didn’t help. I am on PHP7.0.x.x.
Did you re-write the theme to be v8 compatible? The same theme/blocks/etc will not work from v5.6 to v7+
Also v8 is EOL so you should probably be starting with v9.