My personal rice of NextCloud. It should make it more professional looking as I hate the overly circular look of every element.

nextcloud-theme.css

/* Expand page content and remove circular border. This was introduced in NX 26
which added the theming options and updated the look. But this made it so there
is a big margin on the entire page.
Content of the app (not header) */
#content-vue, #content {
  margin-left: 0;
  border-radius: 0;
  left: 0;
  right: 0;
  margin-right: 0;
  width: 100%;
  margin-bottom: 0;
  height: calc(var(--body-height) + var(--body-container-margin));
}


/* Redefines some common elements such as different size buttons
to not be pill-shaped. They will instead be more square.
"#app-navigation:not(.vue)" is for the sidebar as it overrides the :root. */
:root, #app-navigation:not(.vue) {
  --border-radius: 4px !important;
  --border-radius-large: 8px !important;
  --border-radius-pill: 6px !important;
  --body-container-radius: 8px !important;
}

/* Contacts Page Buttons: Make them not pill shaped */
.list-item[data-v-65f2f7ad], .button-vue {
  border-radius: var(--border-radius) !important;
}

/* Hide the dashboard button from the app menu, it's redundant because you can just click the logo. */
.app-menu-entry[data-app-id="dashboard"]{
  visibility: collapse;
}

/*
  _________.__    .___          _____                       
 /   _____/|__| __| _/____     /     \   ____   ____  __ __ 
 \_____  \ |  |/ __ |/ __ \   /  \ /  \_/ __ \ /    \|  |  \
 /        \|  / /_/ \  ___/  /    Y    \  ___/|   |  \  |  /
/_______  /|__\____ |\___  > \____|__  /\___  >___|  /____/ 
        \/         \/    \/          \/     \/     \/       
*/

#side-menu{
  box-shadow: rgba(0, 0, 0, 0.4) 0px 25.6px 57.6px 0px, rgba(0, 0, 0, 1.0) 0px 4.8px 14.4px 0px;
}

#side-menu.open {
  animation: sidemenu-open 0.2s ease-in-out;
}

@keyframes sidemenu-open {
  from {
    translate: -100px 0;
    opacity: 0;
  }

  to {
    transform: 0;
    opacity: 1;
  }
}

/*
  _________       __    __  .__                      
 /   _____/ _____/  |__/  |_|__| ____    ____  ______
 \_____  \_/ __ \   __\   __\  |/    \  / ___\/  ___/
 /        \  ___/|  |  |  | |  |   |  \/ /_/  >___ \ 
/_______  /\___  >__|  |__| |__|___|  /\___  /____  >
        \/     \/                   \//_____/     \/ 
*/

/* Make the theming CSS text-box in settings larger */
textarea#theming-customcss-input {
  min-height: 650px;
}

/*
___________              __    ___________    .___.__  __                
\__    ___/___ ___  ____/  |_  \_   _____/  __| _/|__|/  |_  ___________ 
  |    |_/ __ \\  \/  /\   __\  |    __)_  / __ | |  \   __\/  _ \_  __ \
  |    |\  ___/ >    <  |  |    |        \/ /_/ | |  ||  | (  <_> )  | \/
  |____| \___  >__/\_ \ |__|   /_______  /\____ | |__||__|  \____/|__|   
             \/      \/                \/      \/                        
*/

/* Increase text editor width. */
.editor__content, .text-editor__content {
  /* We either set this to 80% of the parent width or
  we set this to 670px which was the default.*/
  --text-editor-max-width: max(80%, 670px);
}
 
/* Add a small border around images to make them easier to see. */
  .text-editor__content .image__view {
  border: lightgray 1px solid;
}


/*
_________        .__                     .___             
\_   ___ \_____  |  |   ____   ____    __| _/____ _______ 
/    \  \/\__  \ |  | _/ __ \ /    \  / __ |\__  \\_  __ \
\     \____/ __ \|  |_\  ___/|   |  \/ /_/ | / __ \|  | \/
 \______  (____  /____/\___  >___|  /\____ |(____  /__|   
        \/     \/          \/     \/      \/     \/       
*/

/* Toggle sidebar button */
.app-calendar .app-navigation-toggle-wrapper {
  border: lightgray 1px solid;
  border-radius: var(--border-radius-pill);
  -webkit-box-shadow: 2px 2px 5px 0px rgba(0,0,0,0.5);
  -moz-box-shadow: 2px 2px 5px 0px rgba(0,0,0,0.5);
  box-shadow: 2px 2px 5px 0px rgba(0,0,0,0.5);
}


/*
   _____               .__        
  /     \  __ __  _____|__| ____  
 /  \ /  \|  |  \/  ___/  |/ ___\ 
/    Y    \  |  /\___ \|  \  \___ 
\____|__  /____//____  >__|\___  >
        \/           \/        \/ 
*/
.music-navigation-item {
  --border-radius-rounded: var(--border-radius);
}