Logo updates (#128)

* add new logos and remove old ones per NSF NCAR branding guidance

* missed a line

Author: kafitzgerald

_static/images/logos/GeoCAT_nsf.svg

@@ -257,7 +257,12 @@
 
 # The name of an image file (relative to this directory) to place at the top
 # of the sidebar.
-html_logo = '_static/images/logos/GeoCAT_nsf.svg'
+html_theme_options = {
+    "logo": {
+        "image_light": '_static/images/logos/NSF_NCAR_light.svg',
+        "image_dark": '_static/images/logos/NSF_NCAR_dark.svg',
+    }
+}
 
 # The name of an image file (relative to this directory) to use as a favicon of
 # the docs.  This file should be a Windows icon file (.ico) being 16x16 or 32x32