I am fairly a novice when it comes to branding SharePoint. We have have a SharePoint 2016 site that I have inherited and I am being asked to change the overlay of the built-in theme we are using to let more of the background image show through. I have read that the default overlay is .85. I can't seem to figure out how to change this percentage. I can see in our custom masterpage where I believe it is being done in the s4-workspace calling the ms-core-overlay. When I look at it in designer it brings up the corev15.css. if i edit the .85 for the overlay nothing changes. Just not sure where to go from here. I seem to go in circles with the info i find online. I hate to say this but exact instructions if possible would be super helpful. =)
There are a couple of ways to do this. If you want it globally you need to determine the exact id/class of the portions you wish changed in the css. Then you need make a renamed copy of that css for backup purposes. Then modify the file. If it does not take, there is another file somewhere that is overwriting the change. You can use F12 on MS Edge or IE 11 to check the classes, styling, etc.
Alternatively, if this is for a specific page and you are not trying to change everything globally, you can write a short CSS stylesheet using a Script Editor Web Part for the page and include the changes there using !important before the ;
Without seeing the exact css and knowing which part you are trying to change, I cannot offer code snippets for you to use.