Add color variables of texts for better accesibility (#7125)

* Add variables for text colors

* Change variables in sass files

* Apply text color variables for recently added colors

* Fix text colors of emoji mart anchors

* Fix text colors of search__input

* Fix text colors of text area of compose-form

* Fix icon colors of privacy dropdown and modal

* Inverted icon colors by classname

* Change variables in boost.scss

* Change action-button-color

* Fix text colors of pre-header

app/javascript/mastodon/features/ui/components/actions_modal.js

@@ -27,7 +27,7 @@ export default class ActionsModal extends ImmutablePureComponent {
     return (
       <li key={`${text}-${i}`}>
         <a href={href} target='_blank' rel='noopener' onClick={this.props.onClick} data-index={i} className={classNames({ active })}>
-          {icon && <IconButton title={text} icon={icon} role='presentation' tabIndex='-1' />}
+          {icon && <IconButton title={text} icon={icon} role='presentation' tabIndex='-1' inverted />}
           <div>
             <div className={classNames({ 'actions-modal__item-label': !!meta })}>{text}</div>
             <div>{meta}</div>