Some really quick hacked together Stylish styles for Lemmy, sort of in the style of old.reddit.com

h5, .h5{
  font-size: 1rem !important;
}
.post-title h5{
    font-size:0.9rem !important;
}
.btn {
  font-size:0.75rem !important;
}

/* main layout */
#app > div > div > .container-lg {
    max-width: 100% !important;
}
#app > div > nav {
    max-width: 100% !important;
}
#app > div > div > .container-lg > div > aside {
    font-size:0.8rem;
}
#tagline {
    background-color:rgb(2, 128, 96);
    border-radius:0.5rem;
    border:solid 1px rgba(255,255,255,0.7);
    padding:0.25rem 0.5rem;
    font-size:0.8rem;
    margin-bottom:0.5rem;
}
#tagline p{
    margin-bottom:0;
}
#sidebarContainer h1,h2,h3,h4,h5,h6{
   font-weight:bold;
}
#sidebarContainer h1{
   font-size:1.3rem;
}
#sidebarContainer h2{
   font-size:1.25rem;
}
#sidebarContainer h3{
   font-size:1.2rem;
}
#sidebarContainer h4{
   font-size:1.15rem;
}
#sidebarContainer h5{
   font-size:1.1rem;
}
#sidebarContainer h6{
   font-size:1.05rem;
}
/* post index page */
.post-listings hr.my-3, .person-details hr.my-3 {
   display: none;
}
.post-listing{
    background-color: rgba(255,255,255,0.023);
    margin: 0.4rem 0 !important;
    padding: 0.3rem 1rem !important;
    border: solid 1px rgba(255,255,255,0.2);
    border-radius:0.5rem;
}
.post-listing .post-container > div:first-child{
    min-width:70px;
}
.post-listing .thumbnail{
    width:7rem;
}
.post-listing .offset-sm-3{
    margin-left:180px;
}
.post-listing .person-listing picture img {
    width: calc(var(--bs-body-line-height) * 0.7rem);
    height: calc(var(--bs-body-line-height) * 0.7rem);
}
.post-title h1{
    font-size:0.9rem !important;
}
/* post listing middle row (e.g. @bob to Whatever English * 20 hours ago ) */
.post-container > .col > .row > .col > div.small{
    font-size:0.8rem !important;
}
/* post site domain */

.post-container > .col > .row > .col > p.small{
    font-size:0.8rem !important;
}
/* person profile */
.person-details .comments > .comment {
    margin-left:0 !important;
}
.person-details .comments {
    margin-bottom:0 !important;
}
.person-profile > .row {
    display: flex; 
}
.person-profile > .row > .col-md-8 {
    flex-basis:100%; /* nothing is showing up in the .col-md-4 sidebar right now? maybe temporary issue? */
}

/* comments general */
.comments .comments {
    border-left: solid 1px rgba(255,255,255,0.15);
    padding-left: 1rem !important;
    border-left-width:6px !important;
}
.comment {
    margin-left: 1rem !important;
    background-color:var(--bs-body-bg);
}
.comment .md-div p {
    font-size:0.825rem;
}
.comment .comment-node {
    background-color:rgba(255,255,255,0.028);
    border:solid 1px rgba(255,255,255,0.3);
    margin-bottom:3px;
    padding:0.25rem 0.5rem !important;
    border-radius:0.5rem;
    border-left-width: 3px !important;
}
.comment .comment-node .flex-grow-1 {
    display:none;
}
.comment-node > .ms-2 { 
  margin:0 !important;
}

/* post page */
.post ul.comments > .comment {
    margin-left: 0 !important;   
}

/* navbar */
.navbar-light .navbar-nav .nav-link {
  background-color: rgba(255,255,255,0.03);
  margin-right: 0.5rem;
}

/* bottom row of post listing right (comments, star, etc.) */
.post-listing .row .col-12 .d-flex:last-child{
    margin-bottom:0 !important;
}
.post-listing picture img.rounded-circle{
    width:1.25rem;
    height:1.25rem;
}
.post-listing .d-none .row .col-sm-2 {
    max-width:150px;
}
.post-listing .d-none .row .col-sm-9 {
    display:flex;
    align-items:center;
}
#app > .mt-4.p-0.fl-1 {
    margin-top:0rem !important;
}

I'll probably keep tweaking these, stay tuned.

edit 2023-07-02 - updated for new layout changes. Still need couple more classes in lemmy-ui for some big containers & comment stuff. Stay tuned and maybe I can get this in the main repo.