Unread email are bold black read messages are unbolded text, email that are defined a new unseen are blue. Clearly you are seeing them is you notice the colour change so they are no longer unseen.

Hi Matt,

Thanks for your reply but I want subject Line colour Blue until I open the email even though it is seen.

Thanks,

Yatin

In Thunderbird settings - general, you could set a longer time to pass before messages that have the focus are considered read.

Hi Scooter,

I did this but does not work.

Thanks,

Yatin

In what way does it not work? Does it not keep a message marked as unread for the duration that you set?

It makes the message bold but both sender and subject are in black color.

I'm sorry that I'm not understanding. I thought that you wanted to see unread messages clearly distinguished from read messages. What I suggested does that. Apparently, what you want is for messages to remain marked as unseen even after you have seen them. Matt is right, above.

I do not know a way to treat seen messages as unseen.

Can my suggestion be used to accomplish almost the same thing?

Or set a filter to mark every incoming message as starred, and turn off the star after you have processed the message.

How to do it? Can you please tell me the steps?

Menu: Tools - Message Filters

See how far you get on your own and ask if you want help with the rules.

Good luck.

Thanks for your suggestions.

"The issue I'm encountering is that both the sender's name and the subject line are displayed in bold black, making them difficult to distinguish. I would prefer the subject line to appear in blue and the sender's name in black. Additionally, when an email is opened, both the sender's name and subject should be in black, but with a noticeable difference in font size—where the sender's name is slightly larger than the subject. This functionality is similar to what Outlook Desktop offers."

I think most of what you want can be achieved with css. Here is an example, and there are many others on this and related forums:

https://support.mozilla.org/en-US/questions/1447610#answer-1653361

Thanks very much. It does what I want. I appreciate your help.

Thanks again.

Hi Matt, Thanks for your reply but I want subject Line colour Blue until I open the email even though it is seen. Thanks, Yatin

I am not aware of a theme to enable you to have any colour other than black for unread messages. But you are welcome to look through them yourself. https://addons.thunderbird.net/en-US/thunderbird/search/?q=theme&appver=&platform=

Please be sure to note the vesions themes work wiuth, there are very few for current releases.