In the Magento 2 DevDoc it says
To include a CSS file, add the <css src="<path>/<file>" media="print|<option>"/> block in <head> section in a layout file.
However it doesn't say how we can influence the order of our CSS files. And if we add CSS files this way in a custom theme which has a parent theme the result is that the files are added quite at the top of the <head> section with a lot of other CSS files below them, meaning their priority is quite low and we cannot easily override rules from the parent theme or extensions.
There were similar problems in Magento 1 and there were workarounds. Some were more clean others less.
What is the best way to order a CSS file of a custom theme at the bottom of <head> in Magento 2 - if possible complying the Magento 2 layout guidelines for custom themes?