Good planning is half the battle. If you want the front rounded off - and I'm assuming here you mean the corners rounded off, but I suppose you could also mean rounding off the front edge of the shelf - then it will have to protrude, as handymac suggests by a couple of inches perhaps. Do you want it to protrude? If you do, then you have to see what widths your floorboards are and make sure that the outer board will have enough support in the event that someone leans or hangs on it. If the widths work out so that the outer board does not have enough support, then it may look best to trim down the width of the inner board, so you can still use a full width for the outer board. Is there solid wood beneath the drywall on this ledge? You need to know how you are going to hold it down and hold the boards together. Handymac is suggesting glue. Me, I like to be able to take things like this apart so that it is easier to repaint the wall, so I would use screws - either round-head or oval-head brass screws, to make them a feature, or carefully countersunk flat-head screws. If there is not solid wood under the drywall... hmm, the ledge at least has to be framed so you could install 16" long pieces cross-wise instead of going length-wise. Or go length-wise, but make sure your inner and outer boards can be screwed. If the shelf does not protrude, holding it down is less of an issue. Heck, it could just be sitting in there - given the profile of the opening, it would stay in well, and frankly, I think it would look odd if it did protrude from this framed opening (and I think I would like a square corner too). Think of the shelf as a giant cutting board. You could assemble it out of floorboards or other lumber and make it removable as a whole. (you have to be able to get the wider back part through the front opening of the space, on a diagonal - check that you can do this). I've never worked with floorboards, but I'm thinking you may not even have to attach them to each other if you pre-finish them and if the outermost board hooks behind the opening's framing. The outer board would hold all the others in. Mac and I are both assuming your floorboards are solid wood, not laminate. If they're laminate (or if you use the boards cross-wise), you will need to conceal the edge as you would for plywood, either with iron-on edge banding as for RRMI's suggestion, or with purchased wood trim. If using the flooring proves too complicated, then solid lumber is available in a variety of sizes, and you should be able to buy dimensions that would add up to 16" or whatever you want. Say, two 1x6s, which would be actually 5 and 1/2 wide, and two 1x3s, which would be 2 and 1/2. I'm sure you can also buy a board that is the full size, but it may not be as stable (staying flat), it may not be finished lumber... or not precisely the width you need. Again, if you need to add a fiddly piece to make the width right, fit it at the inside edge. Cutting the pieces precisely to length (and width) will be tricky and in case you blow it (as I am always at risk of doing), you could plan to use corner moulding atop the ends or along the back to conceal any unevenness. To give any more specific advice, we would have to know more about your tools and your woodworking skills. Karin L PS your photos don't show for me so I could only magnify the thumbnails.

I agree with hatetoshop, just move the silver candles in front of the ornate stands. Then a colorful platter where the bowl is (maybe with some of the red) and IMO your done. Looks good

Could we see the whole space and please get rid of 1/2 those pillows on the sofa. IMO before doing any decor on those shelves really think about what you have that is worth displaying , not just a bunch of stuff to fill them. I look at the examples above and all I see is a bunch of things really not that special and the amount of dusting to keep it perfect. I do however agree a dark background .BTW the vase on the coffee table is way too tall.

There is nothing wrong with what you have on the shelves, other than the objects blend in with the background. IMO that unit is too small in scale for the size of that wall.