Hmmm, yeah, you're right. Modern Cards purposely uses the absolute minimum size when it comes to splitters (2 px) without any triangle arrows to collapse/expand it. Looking at the skin's main.xml I see this (along with the comments);
<!-- NOTE: The default splitter takes up to much space. Yes, it's a bit more difficult to hit with it at 2 px, but it's still usable. In this skin it's matching the tree background color. There's one "issue" with this though. Views where you've got a pane to the side of the list, you'll get a 2px dark border that doesn't match the list background color. There's nothing that can be done about this, short of removing the splitter completely. -->
<SPLITTER>
<Data DefaultSize="1" />
<Entry Name="HorizontalBackground" Bitmap="Splitter_Background_H.png" Margins="9,1,9,1" Columns="398,?-Flex,398" />
<Entry Name="VerticalBackground" Bitmap="Splitter_Background_V.png" Rows="23,?-Flex" />
<Entry Name="HorizontalHandle" Bitmap="Splitter_Horizontal.png" Margins="2,1,1,1" DrawMode="REGION_E_HTILE,REGION_E_VTILE" />
<!-- TODO: Is there any way to make sure the "triangle" control doesn't show up in a splitter, without having to make "VericalHandle" a stupidly tall image? There probably is, but how?! -->
<Entry Name="VerticalHandle" Bitmap="Splitter_Vertical.png" Margins="1,2,1,1" DrawMode="REGION_E_HTILE,REGION_E_VTILE" />
</SPLITTER>
I agree with the part about the default splitter taking up too much space, but it looks like the skin doesn't (or most likely can't) account for split view being used. Also looks like this could be an unfinished portion of the skin, not sure, since the triangles are purposely blocked/not used. Unlike the tree splitter, looks like you can't double click on the split view splitters to collapse/expand them AND the split views when collapsed don't appear correctly. Not sure if there's an easy way around this, short of restoring larger splitters with the triangles which *could* look terrible with this skin. Sadly, this is likely outside of my current ability to fix/workaround as I've yet to work with splitters.