Ability to disable dark mode

[james-nash]

Is your feature request related to a problem? Please describe. The Cycle 7 website includes various images and diagrams that don't play nice with dark mode. The team doesn't currently have bandwidth to create dark alternatives for them, so rather than delivering a broken dark mode, they'd like to disable it entirely for now.

Describe the solution you'd like A build-time option via a SASS config variable would be a workable solution for this.

[james-nash]

☝️ FYI: @danbull & @patrickbower