Hide all the gamification and other elements,o ther than

duolingo-focused.css

[data-test="skill-tree"] > div:nth-child(3) > div:nth-child(1), /* failed levels and get plus reminders */
[data-test="skill-tree"] > div:nth-child(3) > div:nth-child(3), /* practice button */
#root > div > div:nth-child(4) > div > div > div:nth-child(1),  /* all gamification elements */
#overlays,                                                      /* random product offers */

/* Top Navigation */
#root > div > div:nth-child(3),                                 /* entire navigation bar */
[aria-current="page"],                                          /* nav - learn button */
[data-test="stories-nav"],                                      /* nav - stories */
[data-test="discussion-nav"],                                   /* nav - discussion button */
[data-test="shop-nav"],                                         /* nav - shop button */
[data-test="courses-menu"],                                     /* nav - courses */
[data-test="crown-menu"],                                       /* nav - crowns */
[data-test="streak-menu"],                                      /* nav - streak reminder */
[data-test="lingot-menu"],                                      /* nav - lingot counter */
[data-test="profile-dropdown"],                                 /* nav - profile icon */

/* Top Navigation - "More" menu item dropdown */
[data-test="more-nav"],                                         /* more button */
[href*="events.duolingo.com"],                                  /* more - events */
[data-test="podcast-nav"],                                      /* more - podcast */
[data-test="schools-overflow-nav"],                             /* more - schools */
[data-test="vocab-nav"]                                         /* more - words */
{
    display: none;
}

#root > div > div:nth-child(3) > div:nth-child(2)
{
/*    max-width: 500px;                              /* squeeze nav items together after hiding some */
}

Apply this using Stylus to the URL "https://www.duolingo.com/learn".