Is browsing and downloading community themes also an issue?
Can you access this file in a browser?
https://raw.githubusercontent.com/obsidianmd/obsidian-releases/master/community-plugins.json
Some things to try that have helped people in the past:
- use a different network (phone hotspot, pocket wi-fi, friend’s wi-fi, etc.)
- use a VPN
- change your DNS settings and restart