made alerts use red with transparent theme red. some UI improvs

src/App.scss

src/_variables.scss

src/components/login_form/login_form.vue

src/components/post_status_form/post_status_form.vue

src/components/registration/registration.vue

src/components/style_switcher/style_switcher.vue

src/components/timeline/timeline.vue

src/components/user_finder/user_finder.vue

src/services/style_setter/style_setter.js