Indeterminate notification preference toggles have insufficient color contrast

Steps to reproduce

1. Go to the Notifications section of a user profile, or of the wiki administration.
2. Disable only some of the event types of an application, e.g. disable the "Alert" toggle of the "A page is modified" event type of the "Pages" application.

Expected

The application-level toggle, which is now in the indeterminate (mixed) state, is readable and meets the WCAG 2 AA minimum contrast ratio of 4.5:1.

Actual

The indeterminate toggle is drawn at 50% opacity, so its ON/OFF texts are barely readable:

• ON: #e6eff6 on #99bddb, ratio 1.69:1
• OFF: #7e7e7e on #f7f7f7, ratio 3.78:1

The indeterminate state also looks the same as a disabled switch, so it suggests the toggle can't be used.

Notes

• The opacity comes from the Bootstrap Switch 3.3.4 stylesheet (css/bootstrap3/bootstrap-switch.css), which uses the same opacity: 0.5 rule for .bootstrap-switch-disabled, .bootstrap-switch-readonly and .bootstrap-switch-indeterminate.
• The state is set in XWiki.Notifications.Code.NotificationsApplicationsPreferencesMacro with bootstrapSwitch('indeterminate', true) when the event types of an application are mixed.
• The automated WCAG checks report it as a warning in NotificationsSettingsIT#globalAndOtherUserSettings and #customFiltersAndLiveData, see e.g. the wcagWarnings.txt artifact of Environment Tests master #1397.
• Possible fix: override the opacity for the indeterminate state in XWiki.Notifications.Code.BootstrapSwitch and give that state a visual cue that keeps enough contrast.