Trying to make a block semi-opaque - what am I doing wrong?

I am trying to make just one shape block semi-opaque using CSS. This is the code I am using:

#block-yui_3_17_2_1_1695040546101_2860 {opacity:.7}

I've tried using % value instead, and neither do anything. I'm using the chrome block ID identifier to get the block ID, so what am I doing wrong?

@SandNotOil Would it be possible to share your site URL so I can take a look? Thanks! 

It's a trial site that I am setting up, so can't share the URL

Please post the URL for a page on your site where we can see your issue.

A link to the backend of the your site won’t work for us, i.e. a url that contains /config/.

Please set up a site-wide password, if your site is not public and you've not already done so.

Post the password here.

Adding a site-wide password does not allow anyone to alter your site. It only allows those with the password to see your site.

Please read the site-wide password and how to share a link documentation to understand how they work.

We can then take a look at your issue.

You may find How to post a forum question post useful.