i cannot see a way to exclude the date from the white background. It seems to be a part of the block that includes the title. I did enlarge the left margin a bit for the white box behind the text of the article. I included the Title with a white background. That one extends over to the left over the date and I have made it readable on the white background.
#content .post-entry {background-color: #fff;}
#content .post-entry {color: #000 !important;}
.post-entry {padding: 20px;}
.post-title h1 {background-color: #fff;}
.post-date * {
display: block;
padding: 0 15px;
position: relative;
background-color: #fff !important; color:#000;}
or a slight variation:
#content .post-entry {background-color: #fff;}
#content .post-entry {color: #000 !important;}
.post-entry {padding: 20px;}
.post-title {background-color: #fff;}
.post-date * {
display: block;
padding: 0 15px;
position: relative;
background-color: #fff !important; color:#000;}
Look at these and see if one is tolerable.