fix: two issues found in post-merge review of PRs #82 #83 (#84)

- routes.py /api/git-info: get_session raises KeyError on miss, does not
  return None -- wrap in try/except KeyError to correctly return 404
  (PR #82, api/routes.py line 222)

- style.css ctx-bar used undefined --teal CSS variable -- replaced with
  --blue which is defined in :root and fits the existing color palette
  (PR #83, static/style.css)

Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>

api/routes.py

@@ -219,8 +219,9 @@ def handle_get(handler, parsed):
         sid = qs.get('session_id', [''])[0]
         if not sid:
             return bad(handler, 'session_id required')
-        s = get_session(sid)
-        if not s:
+        try:
+            s = get_session(sid)
+        except KeyError:
             return bad(handler, 'Session not found', 404)
         from api.workspace import git_info_for_workspace
         info = git_info_for_workspace(Path(s.workspace))