Magento 2 – How to Change Locale Code for Theme

According to this Magento 2 Github issue the functionality for this has not yet been implemented although they are in discussions about adding it.

As a workaround you can apparently delete and uninstall the luma theme (see instructions in my quote below) from the vendor directory and it will skip it. You may need to clear var/view_preprocessed, pub/static, and clear you caches for it to work though.

Comment from Alan Kent is on that github issue:

(If you want a short term hack, you can delete the Luma theme directory under 'vendor' so Magento does not see it any more. Make sure you run the CLI to uninstall the theme as well so it disappears from database etc. http://devdocs.magento.com/guides/v2.0/install-gde/install/cli/install-cli-theme-uninstall.html. Yes, we need a better solution, but it may remove some immediate pain.)

I just had this issue, too, and found that it is because of the order by which the css and less are compiled. You need to use the style.less file. Copy that from the blank theme and I noticed that the .css files were then auto generated with my custom css directory. However, any changes that would need to be made in the styles.less could mostly likely be made in the _theme.less. This is how I ultimately solved the issue. Within the _theme.less you can make specific css changes or make changes to variables like colors and such.

Here are a couple of examples:

*Overriding colors
@color-darkslateblue: #2d5986;
@color-bluehover: #426992;
@color-floralwhite: #f8f8ee;
@color-blue1: @color-darkslateblue;
@color-blue2: @color-bluehover;

/*customizations*/
.nav-sections {
  background: @color-darkslateblue;
}