Wow, it looks better than Steemit. This dark mode doesn't kill my eyes like green, but is a steampeak secure?
Yes it should be fully secure, I personally use it with the Steem Keychain plugin on Firefox, but you can also use SteemConnect.
The website itself does not get access to your private posting keys etc.
I think that you do not need to give it your master password even, so even if somehow your private posting key gets compromised, you can still change it as long as you have your master password.