Replace the json nav background color with the real one

fixes: CNVS-21432

This brings our secondary blue color back to the darker blue we had originally
had in there.

Test Plan:
- Turn on new ui
Main navigation background:
- Was: http://cl.ly/image/0i2v0E0v0D3u
- Is now: http://cl.ly/image/0u2w270h0b0j

Change-Id: Ib729395d4449f949d190cdd7b59c70a6da2a623e
Reviewed-on: https://gerrit.instructure.com/57862
Reviewed-by: Jennifer Stern <jstern@instructure.com>
Tested-by: Jenkins
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Jennifer Stern <jstern@instructure.com>

app/stylesheets/brandable_variables.json

@@ -9,7 +9,7 @@
     "human_name": "Secondary Color",
     "variable_name": "ic-brand-secondary",
     "type": "color",
-    "default": "#5b6c79"
+    "default": "#34444f"
   },{
     "human_name": "Primary Button",
     "variable_name": "ic-brand-button--primary-bgd",