Thank you for your kind words.
That is a good question and that is exactly what you have to decide.
XF has a built-in setting, where you can easily change any color on your forum, so if you find a theme you like, you could match the colors on your website easily. Also with little html/css changes you could match more stuff (but it is limited).
But what you can't do is to match the look of the forum with your website 100%. For that you need a custom theme development.
But again, let's say you have a forum/site about gaming. There are a lot of gaming themes out there. Why need to re-invent the wheel (custom development), when you could get one of those existing ones and match it with your website as best as you can (colors and html/css changes)?
In other cases, when it is a unique thing, it will not be possible. You need customized theme.
(You can find free/paid themes
here).
I suggested to get an existing theme, because you were asking about the updates. That is exactly what you need to consider.
You get a custom theme, which means on the upside it matches with your site, but on the downside you need to worry about updates (although for XF 1.5 not anymore).
Or you get an existing theme, on the upside you don't need to worry about updates, on the downside it may be not representing your website as best as you want.
There are so many themes available and in most cases there is always 1 or 2 themes people find fitting for their niche, without needing a custom development. You must make the call.
For example you can find
here many people showcasing their forums. Each have a niche but most of them go with available themes. You can see how much you can make it fit with available themes.
Frankly said, it also depends on your budget. The big boards tend to have customized themes (you can find a good list
here). They have the budget to go with that way. You have to know what you want to go with. There are handful of good theme creators here, who all can do great custom work for you and who make sure that the styles they create can be updated easily. Maybe ask for a quote and ask for the support they would give. Then decide what is best for you.
Wish you the best, good luck.