fix(style): light theme fixes/improvements

[matmen]

Closes #673

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Screenshots of the new styling (light and dark theme):

[pedrolamas]

For the AppNotificationsMenu, it's easier to just add the light--theme entries:

after this:

  ::v-deep .app-notifications .v-list-item .v-list-item__subtitle.notification-timestamp {
    color: rgba(255, 255, 255, 0.47);
  }

add this:

  .theme--light ::v-deep .app-notifications .v-list-item .v-list-item__subtitle.notification-timestamp {
    color: rgba(0, 0, 0, 0.47);
  }

And after this:

  ::v-deep .app-notifications .v-list-item .v-list-item__subtitle.notification-description {
    font-style: italic;
    color: rgba(255, 255, 255, 0.60);
  }

add this:

  .theme--light ::v-deep .app-notifications .v-list-item .v-list-item__subtitle.notification-description {
    color: rgba(0, 0, 0, 0.60);
  }