ISBAT Cover Block: Improve block styles

**Is your feature request related to a problem? Please describe.**
Currently, the cover block offers two alternative styles - bottom and top wave. The extra styles are welcome, but these particular ones are a bit impractical. The waves are very dynamic, which means they require extra padding and end up being really tall if you want to put text inside the cover block.

### Describe the solution you'd like
My proposal is to modify the current styles and make the waves gentler. Here's roughly the shape I had in mind:


For comparison, here's how this cover block style looks like now (same resolution as the example above) I had toadd a 100px high spacer above the text here, otherwise the text would be right next to the edge:

Would you consider making this kind of change? Would it affect sites where those block styles are already in use?