You can change the Blogger template opacity/transparency level easily by changing a few numbers in the code. Here's how to do it:
Click on the Layout: Edit HTML tab. Then, search for the code below:
margin: 0px 0px 0px 0px;
padding: 0px 0px 3px 3px;
border-right: 1px solid $blogBorderColor;
Change the transparency level by changing the numbers highlighted in red. Vary the numbers between 0 to 100 for the line 1, or 0 to 1.0 for the lines 2 and 3. All the numbers must be at the same level, for example 75 for line 1 is equal to 0.75 for lines 2 and 3. The lower the number, the more transparent your blog is. If you don't want it to be transparent, set the values to 100, or 1.0.