Design themes

I chose a design theme and made changes using the design ideas, which essentially expanded the patterns further across the slide. I subsequently changed the theme colours and used that as a base, adding pictures and text. Once I'd finished I realised the slide wasn't the right size for what I need, but when I try to re-size it regardless of whether I choose 'maximise' or 'ensure fit' the background theme changes significantly. I tried to get round this by creating a slide in the right size first and then using the design theme but then the design ideas don't show as an option meaning I haven't been able to recreate the same design that I so like. Is there anything I can do to get around this?