Things to consider when creating a 50th Anniversary Scrap/Memory Book

What will they be eating and drinking? You already mentioned champagne but keep in mind what kind of party they throw - picnic, restaurant?

Keep your backgrounds the same for each page, but you can vary how much of the background shows. Half? 3/4?

Don't be afraid to use the same items on many pages. It will help with cohesiveness.

I’ll try to find examples and get back to you.

Realize that whatever you do, they will be thrilled.

That color scheme may be difficult to work with as the dark color for the background can be intimidating. I would use the dark gray sparingly and stick to yellow/gold as the main color of the background. I like the idea of matching the sign-in book.  When you use a darker background, it probably will be necessary to outline or frame the photos in a lighter color to call the eye back to them.  But then again, if the photos are all lighter in overall look, the dark background will work well.

Still looking for some examples of how to vary background. I don't have a lot of themed multi-page layouts.

Maybe these duck layouts are a good example? I carried the color scheme from page to page without using the same background for each.  That yellow one kinda stands out, but maybe as a title page, doing one with an introduction of the color scheme, will work.  I grabbed that rust color from the mulch used in the planting bed she was nesting in.  After you get the photos of the event, a coordinating color may jump out at you.

If you are there at the beginning, maybe you can grab some photos of the venue before people 'mess things up' - 
If there are centerpieces, try to grab a photo of one of them to use as a decorative item on a page.

Often, when I don't have enough background papers in a scrapbook kit, I just use a brush on a solid background.  (brush on a different layer so it can be edited or deleted as needed). Don't be afraid to look through kits that have absolutely nothing to do with the subject matter.  Sometimes there are background papers or items in a kit that you'd never think to look at.  For me, the hardest part of creating a book is finding the kits to use.

When making your background papers, if you choose a color for your brush that is close to the solid color, it doesn't 'scream' and detract from your photos, however, doing a block of bolder stuff - maybe surrounding one photo - can add interest.  In this case, your photos will want to be the focus of your pages - so don't do bold patterned backgrounds that will draw the eye away from the photos.  When creating your background papers, if you want to draw straight lines of brush strokes, hold down the shift key as you drag.  

There are two schools of thought on fonts.  Some think using the same font throughout the book makes it cohesive, but for me, sometimes a fancy font is a decorative item on the page.  However, for ease of creating, using the same font will take a lot of work out of the page design.

As for 'doodads' - as I like to call them - 
* strips of color with patterns
* tags with text (in this case, describing who is at the party is essential)
* on the title page, start with the honorees photo and the date -
* see if you can get an original wedding photo - 'then and now' is always so interesting
* champagne glasses on the page featuring the toast, food items on the dinner page

Don't be afraid of getting 'cutesy' - this is a fun time - celebrating life - keep it happy and unique

Realize there is so much free 'stuff' on the internet.  Try and find pngs because deleting background from items you wish to use is a pain and just never looks as good as already created pngs.

Also, realize that I'm a minimalist - as you can see from these examples.  I rarely do boldly designed pages - mainly because I'm not good at working with bold patterns.  They always 'scream' at me.  However, that doesn't mean they are wrong - it's just that I'm not comfortable using them.

Hiclipart.com is a great place for png files and they have a good selection of the likes of wedding rings, champagne glasses balloons and frames that might be suitable for your project.

I always prefer using a template- even if the end result looks nothing like the template I began with. It really helps my creativity. Creating some templates from layouts you like is a great idea.

EDITED

Forgot to mention.  That closeup of the color scheme shows you will have excellent colors to work with.  This is going to be such a fun and memorable project.  However, don't over-think your pages - get them done in a timely manner.  Remember - they will be thrilled at getting such a personal and wonderful gift 'created with love' - not many people can do what you can do to create memories.

I have created a folder of pngs that I may use. Thank you!

Sounds like a very fun project and it will be a wonderful gift.

You’ve made a great start, my preference is for the lighter background as I think the photos aren’t competing with the darker background.