Defintion is always a problem in this 'chat room'.
I will try to bring a few things together for you, including some of the answers you have been given.
First, in all sciences the word 'theory' does not mean 'idea' or 'guess', it is effectively a 'folder' where you will find everything about that subject. So 'The Theory of Motion', a theory that began life with Isaac Newton, is where you will find the 'laws of motion', facts about gravity, and some of the information that is also part of the Theory of General Relativity that was founded by Einstein, it will also contain all the research, failed, proven, or otherwise that relates to the motion of objects.
So, the 'Theory of Evolution', will contain all that we know to be true about how life developes, all the information on genetics, all the information on DNA, including chemical structure, physical properties, and how DNA interacts with other compounds. The theory will also contain all the research that has been done over the last 150 years since Darwin, and those things that were 'known' before that time, including failed ideas.
The reason theories do not in themselves ever become 'laws' is two fold, one laws are actually part of theory, and secondly new research and information can be added at anytime, this does not detract from the facts that are already known and proved.
The reality here is that people use the word theory in every day life to mean something that it doesn't, they use it as a clever sounding alternative for the word idea, and that is wrong.
A further point, 'atheists' do not 'believe' is facts, by definition, an atheist is not any different from a theist, a theist believes in a god, deity, or formal religion, an atheist is simply someone that does NOT believe in those things, no 'facts' are required for either position.
What you need to remember is that not all atheists are scientists, and not all scientists are atheists.
You will find that there are scientists, not many and probably very, very few in physics, cosmology, or astronomy, that 'beleive in a god or deity, and there are many people who are religious who accept the facts of science, and are therefore not 'creationists', there is not a one thing or another choice here, it is simply that the debate has become polarised because the loudest voices are the creationists, as their very existence depoends on them trying to cast doubt of what science has already proven as fact.
To sum up, evolution has proven facts behind it, genetics for a start, so whether or not people chose to beleive it, it happens, much in the same way that back in the past (1600's) the Pope at the time wanted to 'believe' the Sun went around the Earth made no difference to the fact that the opposite proposed by Galileo was true.