For first question:

https://newbloggerthemes.com/blog/fix-posted-date-undefined-error-blogger/

For second question, contact me with your site URL.

i juste have two little questions cause I have some problems with the post dates. They don't appear on my blog (on the front page and on the article page, it's the same), only the hours appear. I think it's because the date is in english in this template whereas my blog is in french. How can I fix this ? Cause i tried to look for it by myself but i can't find how to do this.
And another little thing is about the header. I put an image instead of a text for my blog title. But when i put an image, then it's not centered, the logo goes on the left... And again, i don't know what to do to have the image in the center automatically.

So if you can help me with these problems it would be great!
Thanks

you can change it. find this in "Edit HTML":

body,
button,
input,
textarea {
color: #87a2a5;
font-family: 'Vollkorn', serif;
font-size: 20px;
font-size: 2.0rem;
font-weight: 400;
line-height: 1.5;
-webkit-font-smoothing: antialiased;
}

Let's say you need to change font size from 2.0rem to 1.6rem. Change above values like this:

body,
button,
input,
textarea {
color: #87a2a5;
font-family: 'Vollkorn', serif;
font-size: 16px;
font-size: 1.6rem;
font-weight: 400;
line-height: 1.5;
-webkit-font-smoothing: antialiased;
}