Here is my css code for Cards View:

/* bold sender in cards view */
.thread-card-container :is(.sender){
	font-weight: bold !important;
	color: blue !important;
	font-size: 15px !important;
}

/* unread card sender */
tr[is="thread-card"][data-properties~="unread"] :is(.sender) {
      color: black !important;
      font-weight: bold !important;
      font-size: 15px !important;
   }
   
/* indent card subject */
.thread-card-subject-container :is(.subject){
	margin-left: 10px !important;
	font-size: 13px !important;
	color: grey !important;
}

/* unread card subject */
tr[is="thread-card"][data-properties~="unread"] :is(.subject) {
      font-size: 13px !important;
      font-weight: bold !important;
   }

/* selected card bg color */
html|tr[is="thread-card"].selected {
     background-color: lightgreen !important; 
}

Help/Troubleshooting Info, Profile Folder, Open Directory (Win:Folder), close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code, change the numbers and colours as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config. editor, restart TB.

Online css generator

video on how to create a css file (Firefox and TB)

The code to indent and style the Subject makes it distinct from the sender. Choose colours and fonts to suit your colour scheme.

Sorry I can't seem to understand this part: "Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Settings/General/Config." Do I need to do this in the thunderbird client?, because I can't find that setting.

Thank you.

Settings/General, bottom of page, open Config. editor, copy

toolkit.legacyUserProfileCustomizations.stylesheets

into the search, click the toggle icon to make the value true, restart TB.