Good collection of free simple templates wanted

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Dear users, please consult me a bit. My question looks to be simple, but I'm trying to find the solution for a long time.

I'm trying to find good and simple templates for the usage at scientific conferences. There are a lot of templates over the internet, but most of them have vivid backgrounds, photos and a childish look. For me the ideal templates are among the standard set, provided with PowerPoint. These templates are made of geometrical primitives, they also have a bunch of color schemes (important feature!). So, what I need is a collection of templates, which are similar to standard. The fact is that I have used most of standard templates dozens of times, so I just need some more designs. Thanks a lot!