For those who  have to suffer with gmail (like  maybe your workplace
uses gapps) and prefer the 'basic html' interface, but do not use it
because  Google crippled  the interface  by deliberately  omitting a
'select all' button in the mailbox view, userscripts to the rescue:

1. Install [tampermonkey][1]
2. Install this [simple userscript][2]
3. Reload your gmail window/tab and rejoice!

I did not check to see if this setup works in chrome-based browsers,
but I only use Firefox, ymmv.

[1]: https://addons.mozilla.org/en-US/firefox/addon/tampermonkey/
[2]: https://greasyfork.org/en/scripts/372661-select-all-in-basic-html-view-in-gmail/code