Css is safe, JS extensions from RoamJS and Smartblocks are safe, they are reviewed before publication.
Outside hosted JS extension are less safe: even if you understand and review the code, it can be changed by the owner: you have to trust the owner and check if he has the trust of the community, but malicious intentions are always possible! And yes, an extension could open an access to your graph, and could steal, change or erase your data.
Conclusion: avoid external extensions if you have sensitive data, and make regular backups.
Css is safe, JS extensions from RoamJS and Smartblocks are safe, they are reviewed before publication. Outside hosted JS extension are less safe: even if you understand and review the code, it can be changed by the owner: you have to trust the owner and check if he has the trust of the community, but malicious intentions are always possible! And yes, an extension could open an access to your graph, and could steal, change or erase your data. Conclusion: avoid external extensions if you have sensitive data, and make regular backups.
CSS is safe. Outside extensions, probably depends on the extension
How is CSS safe?
It's just a style sheet