It looks very achievable to me.
We prefer using a custom CSS file on the server for each specific Business Space.
Here are the basics that you need to learn to get closer to your goal:
Learn about CSS classes and id's and how different CSS rules are prioritized
Create a .css file in the Custom directory for your Business Space
Learn to use the built-in web developer console (F12) in your browser and the element inspector to see all the loaded CSS for the chosen element for testing things directly or troubleshooting.
These are the most important CSS properties based on your goals:
box-shadow:none;
border:0;
border-radius:0;
width:;
height:;
background-color:;
Good luck with your design! It would be nice to see how your application turns out!