Hi
You can, it's possible, But I don't suggest doing it.
First, I need to mention that you would need to customize the Mashup(s). and other entities Because of this, they would not be upgraded when you upgrade RTPPM : you would have to re-do the entire procedure. It's a risk.
Second, there's an important information to know about RTPPM : the Mashups and most other entities (like the Theme) are part of the Extension, which makes it impossible to edit them directly. To edit them, you need to create a duplicate, associate it to another Project (like PTCDefaultProject), and then you can modify your duplicate. If you want to modify the Theme used in a Mashup, you need to duplicate both the Theme and the Mashup to point to the new Theme.
Now, how to do it. I'm really not a Theme & customization expert. I tested your request and found a behaviour I didn't expect: instead of using the grid widget's Theme, it actually takes the top-level Theme. So what I found is that to modify the colours in the grid, I had to modify the Master called PTC.FSU.COREUI.MenuConfiguration.MasterContainer_MS, change its Theme and change the URL to be http://MY_SERVER_NAME:8080/Thingworx/Runtime/index.html?mashup=PTC.FSU.COREUI.MenuConfiguration.MasterContainer_EP#master=PTC.FSU.COREUI.MenuConfiguration.MasterContainer_MS_Duplicate. And the changes I applied in the theme used by the Master was applied to all grids. In that Master (which is like a Mashup), I had to change the Theme which was PTC.SCA.SCO.MnfgCommonGlobalUITheme by default. To change the selected row's colour, go to in the Theme in Composer -> Styles tab -> Elements -> Grids & Lists -> Grids -> Selected Background.
So in short : duplicate the Theme, modify its Selected Background colour, duplicate the Master "Mashup", modify it to change the Theme it uses, then change the URL you use.
I was also told of an alternative : adding "&__applyThemeName=PTC.SCA.SCO.MnfgCommonGlobalUITheme_Duplicate" at the end of the URL. However I wasn't able to make it work.
