How do I highlight, ie change the color or change to bold, an email account or folder that has new email in it.
As @Gary says you cannot highlight or bold new emails at this stage.
However I take it you have setup rules to automatically move new emails to specific folders and want to see which of your folders have new “Unread Emails” and “How many new emails” in any folder at a glance.
Example my Gmail account account where i have a rule setup to move and incoming specific email address to a folder call “Mal”.
When that new email address arrives, the email moves automatically to the “Mal” folder and also it “does show next to that specific folder” the “New email message number” eg: 1 new message in this case. So i know at a glance that there is new email in that folder. Don’t need to highlight it by color or bold.
See example below.
As the Unread Number then disappears after you have read it, then to remind you later of new emails, suggest to “Flag” any new emails read which will then show in your eM Client "Agenda (on the right) to then read /action again anytime. The Agenda also shows you “The folder location” of the new email.
PS I do not want to use a combined, or unread folder
If you do have the “Unread” combined folder at the top for all of your accounts, You can actually see which folder the “Unread email” is found in (At the top right of the email) as in this example is “Mal”.
You can then also flag that email “from the unread combined folder directly” to then show in the agenda as above.