fix color contrast issues

bigbluebutton-html5/client/stylesheets/toggleSwitch.css

...html5/imports/ui/components/audio/audio-modal/styles.scss

bigbluebutton-html5/imports/ui/components/button/styles.scss

...-html5/imports/ui/components/modal/fullscreen/styles.scss

...ui/components/settings/submenus/application/component.jsx

...html5/imports/ui/components/settings/submenus/styles.scss

...ebutton-html5/imports/ui/components/user-list/styles.scss

...utton-html5/imports/ui/stylesheets/variables/palette.scss