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php bin/magento setup:static-content:deploy --theme Magento/backend --theme Vendor/mytheme

How to deploy a single theme rather than all of them every time
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Some more info re: php bin/magento

If you run this command, and it throws an error at all, it will tell you to clear the cache and generated folders. 

rm -rf var/di
rm -rf var/cache
rm -rf var/generation
rm -rf var/page_cache

Note: these folders will need removing often, if you have any issues removing them please speak up as it's likely to be a permissions issue.