Yes, any major upgrade can kill a theme, but most don't. It depends how much change / work there was. But this should only affect the look & feel, not your data (menus, articles, etc.)
The big changes, theme-wise were between:
- 2 and 3 (general theme revamp)
- 6 and 7 (module layout revamp)
- And the next one is expected between 12 and 13, as we modernize to take advantage of modern browsers. See Version LifeCycle
So between 8.x and 10.x, there is no major change theme-wise. Re-using the theme will likely work. If it doesn't, it's likely a .tpl file, and not a .css, so a theme with fewer modified .tpl files (compared to the default) has a higher chance of not breaking.