I recommend that you create a Theme Sandbox. Create a test post with all the headings and styles you would normally use within your site. View the generated page in the Classic or Default Theme - or whatever Theme most closely resembled the end result you want.
Do a FILE > SAVE AS and save the web page to your site. Start with a single post view and then you can save the front page and other views to get their styling but it helps if you start with the single post view first.
Put the new html file in a folder along with the style sheet. Using a Text ONLY Editor, edit the new file's head to direct the location of the style sheet to the style.css in that folder. Add images and other things you want in that folder or in a subfolder like
/images. Make sure all the links in the test file match the new folder and location.
Then view the page in your browser and it should look great.
Using the Firefox browser, get their Web Developers Add-on extensions and use the Edit CSS feature to bring up a sidebar that allows editing of the style sheet AS YOU WATCH.
It won't show you the background image, but it will show you just about everything else. Edit and make changes and tweak things around, and then copy the CSS to the style.css file and view the page again (total refresh) to see the new changes in place with the background image. Continue to tweak until you get it how you want.
Change the heading in the style.css to be the new name of the Theme you are working on. It doesn't matter what it is called, just call it something different from the themes you already have installed.
When you are ready, upload the new style sheet to a COPY folder of the original Theme you started with and then select it through your Presentation Panel and check out your new Theme.
On the multipost views, you will have to tweak a thing or two, but that's the easy part. You can then do the same thing with the other generated page views and you will have the theme you want.