(Rewritten to reflect decision)

In MDL-43784 we removed all the themes based on base in 2.7, which means we can't use standard.

In Moodle 2.7 and later, we will code that affects UI with:

1. Clean, and
2. Base

Developers and testers can select these themes directly in the URL via ?theme=base or ?theme=clean

MDLQA testing will not require every issue in 2.7 and later to be re-tested on both themes. We'll use Clean for all them. However, we'll also have at least one test that calls for exploratory testing on Moodle using Base theme just to catch any obvious regressions (such as MDL-44844 for example).