Configuration import workflow broken - either defaults not loaded from settings or defaults not set properly

Problem/Motivation

Assuming we start with a valid themename.settings.yml then we should be able to
drush cim, save the settings form followed by drush cex and not see any changes.

However it seems as though there are various settings that don't stick when imported and can only be changed manually.

Steps to reproduce

Create two dxpr subthemes. install both. Make changes to one. Export config. Copy the exported config from one to the one that wasn't changed. retaining any uuids. Import that config. visit the theme settings page and save. export the config. Note differences. Attempt to locate the relevant settings on the theme settings page and manually fix. iterate the last few steps until we see no changes on export.

I found problems with the following:

56c54
< menu_border_size: '1'
---
> menu_border_size: '4'
84c82
< navbar_background_custom: '1'
---
> navbar_background_custom: ''
114c112
< menu_border_color_custom: '2'
---
> menu_border_color_custom: ''
136c134
< headings_font_face: '0Source+Sans+Pro:'
---
> headings_font_face: SF_Segoe_Roboto
138c136
< nav_font_face: '0Source+Sans+Pro:300'
---
> nav_font_face: SF_Segoe_Roboto
140c138
< sitename_font_face: '0Source+Sans+Pro:'
---
> sitename_font_face: SF_Segoe_Roboto
142c140
< blockquote_font_face: '0Source+Sans+Pro:300'
---
> blockquote_font_face: SF_Segoe_Roboto
148,150c146,148
< gutter_horizontal: '100'
< gutter_vertical: '100'
< gutter_container: '30'
---
> gutter_horizontal: '30'
> gutter_vertical: '30'
> gutter_container: '60'
180c178
< block_border_color_custom: '0'
---
> block_border_color_custom: '#eeeeee'
190c188
< title_border_color_custom: '0'
---
> title_border_color_custom: '#eeeeee'
198c196
< block_divider_spacing: '4'
---
> block_divider_spacing: '20'
212c210
< body_line_height: '3'
---
> body_line_height: '1.4'
214,223c212,221
< nav_font_size: '8'
< headings_line_height: '3'
< scale_factor: '2'
< h1_font_size: '8'
< h2_font_size: '8'
< h3_font_size: '52'
< h4_font_size: '38'
< blockquote_font_size: '29'
< blockquote_line_height: '3'
< headings_letter_spacing: '0.3'
---
> nav_font_size: '14'
> headings_line_height: '1.1'
> scale_factor: '1.25'
> h1_font_size: '39'
> h2_font_size: '31'
> h3_font_size: '25'
> h4_font_size: '20'
> blockquote_font_size: '20'
> blockquote_line_height: '1.5'
> headings_letter_spacing: '0'
227,234c225,232
< h1_mobile_font_size: '14'
< h2_mobile_font_size: '14'
< h3_mobile_font_size: '52'
< h4_mobile_font_size: '38'
< blockquote_mobile_font_size: '29'
< divider_thickness: '20'
< divider_length: '18'
< divider_position: '3'
---
> h1_mobile_font_size: '39'
> h2_mobile_font_size: '31'
> h3_mobile_font_size: '25'
> h4_mobile_font_size: '20'
> blockquote_mobile_font_size: '20'
> divider_thickness: '4'
> divider_length: '0'
> divider_position: '1'
236c234
< divider_color_custom: '1'
---
> divider_color_custom: '#eeeeee'