Welcome has a spot for this on the main menu (new for 20.04 - but will available on 18.04 and up!)
For an early test run, check it out via the Welcome PPA, or the beta version of the snap:
snap refresh ubuntu-mate-welcome --channel beta
Also new: Welcome will use the colours from the GTK theme. 
Still ironing out the buttons.
Great news!
Works as expected (both, colors and "Light or Dark") 
Awesome! 
I only have one problem with visibility of the text in dark version in Welcome -> Getting Started -> System Specifications at the bottom of the Basic Storage > Why do I see 2 units?
Assuming Software Boutique is still part of the Welcome, there are some more screens like that, that are harder to read when using dark theme.
I really like this idea and choice to have different colours to chose from.
As much as I love that idea, I have reservations about it. Welcome is suppose to be the portal into Ubuntu MATE assistance and distribution-specific tools. While I am all for everything else matching a user's theme I feel like it would be an onboarding faux pas to make Welcome match, unless it is something an OEM could enable in a configuration file, along with being able to further modify the interface for their own spin on Welcome specific to their brand, for their customers.
Ahh, thanks. I thought I missed a white on white spot! I'll get that fixed.
@tiox I'm not sure what you're on about - OEMs are welcome to fork Welcome to make interface and colour changes. Take a look at Ubuntu Budgie Welcome - it's a fork from a few years back.
It isn't that — I am meaning to say, there is no sense in changing the appearance of Ubuntu MATE's system onboarding materials unless it is an OEM. Ubuntu MATE team is welcome to do what they want; I am fairly certain many will see the change to Welcome as significant and a positive direction for it, I just don't see it in quite the same way.
I see what you mean. Well, this change is small, but it's to complement the colour selection feature. This is for the users 
who switch to Ambiant-MATE-Dark, Radiant-MATE, High-Contrast-Inverse or any of the MATE themes and would expect their applications to follow their preferences.
Yes, but this also means what? That the green accents and branding will be whatever colour as defined in the theme? That's really what I am on about. If it's just to make light things dark and preserve the colour of the Ubuntu MATE "Brand" — as ugly some may think it be — then that is an acceptable direction in my opinion for onboarding.
Not slighting the change, moreso than providing input on preserving the branding, being in logo guideline compliance et cetera. If there is deviation, so be it. Not going to fuss beyond pokes and nudges to keep branding on-track in spite of user input.
Our shade of green is an issue we wontfix.
Just my opinion. I like it!