A room with no windows is a very challenging space: it may seem very dark and uninviting if you don’t style it right. Here are some tips and ideas to do that.
Walls
If you have a windowless room, the main tip to keep it welcoming is to use light shades, and the first thing on the list is the walls. White, light blue or light green will be just fine to style such a space, and using one single color throughout the room will not only blur the boundaries but will also add a trendy feel to the room as monochrome is on top.

Light shades don't mean only white or creamy, you can go for light green or sage that will soothe you.

This small attic bedroom has no windows but looks light enough thanks to the light colors used for styling.

This no window room is done in white and with a light-stained floor, and only the black couch breaks the continuity of the space.

Make a basement home office styled completely in neutrals and add just a bit of black to create a catchy contrast.

If the space is small, using light colors is essential, though you can vary the shades and play with them.

This is a pale blue and white bedroom, and using such colors together with simple furniture designs help the room look larger.

This creamy basement feels like an endless space as it's all done in white, creamy and other light shades. Lots of faux plants enliven the look.

This is a basement cinema that has two main things to make it welcoming: light shades and lots of light incorporated.
Furniture
Stick to light shades while choosing the furniture, too. Don’t choose whimsical and heavily ornated pieces, prefer slender legs, sleek designs and light colors, this will give an airy and breezy feel to the room. If you have a small room, choose double-function furniture to save more space as cluttered rooms always seem smaller.

This all-white living room features tons of light and only a couple of black items to break the monotony of the space.

Even if you think that all-white rooms are boring, just look how such an approach enlarges and illuminates a small no window bedroom!.

Here's how to style a no window room right: a single light shade, lots of lamps and lights and simple modern furniture. (via gatheredky).

Creamy shades and lot of lights are a perfect combo for a no window room. (via quietjoyathome).

This no window space shows that simple furniture with clean lines is always the best idea as it feels airier with it.

This basement living room is full of style, done in creamy shades, with dark-stained wood for a contrast and a lot of light.
Lighting
Lights are the main point when speaking of windowless room designs. Install several layers of light to make the space as illuminated as possible. Pendant lamps and chandeliers, wall and floor lamps, fairy lights and of course hidden lights placed strategically to create an illusion of sunglow. Make sure that every single corner is illuminated at its best.
You can also share light if there’s a next room. Go for glass doors, with usual or frosted glass panels to get some natural light from another space.

Prefer minimal and simple furniture designs that will make your room look less cluttered and airier.

I love how cozy and welcoming this little bedroom looks! Though it doesn't feature any windows, it's still very inviting thanks to the colors and lights.

This basement bedroom features only soft and warm light shades and lots of light from all the sides.

If you have a cinema or living room with no windows, you can always incorporate neon, colorful LED lights and stuff like that.

This light-colored space shows a lot of light incorporated and some potted plants, too, and both these things are essential for no window spaces.

Hidden lights are always a good idea for a no window room, they create an illusion of natural light.
Decor
Mirrors
Mirrors are the secret to success when we are considering small and dark spaces. Reflected light will illuminate and enlarge the room, and that is a perfect solution to apply to a windowless room. Hang several mirrors, go for a single oversized mirror or create a mirror gallery wall, which is so popular now.

Such a large floor mirror in an ornated frame will definitely add to the style and look of the room.

A mirror gallery wall is a trendy idea, and mix and match ones in chic frames will add to the style of the space.

Mirror panels are an alternative to a mirror gallery wall or large mirror, and you can make them yourself repurposing some old pieces.

These large mirror panels on the wall make the space look bigger as they reflect the light from the lamps. (via laurelberninteriors).

This dining room shows off a large mirror on the wall and lots of lamps that make it chic and welcoming.

This mirror arrangement over the stairs is a super cool idea for any room with no windows: just imagine how much light they will reflect.
Decor
As you have no windows, a large wall can be styled as you want. You can cover it with a mural, an oversized wall art, make a focal point and give your space visual depth. Wall art can also be transformative in a space with no windows. By hanging a photo or a framed piece of artwork with a beautiful view, it can look like a window in a windowless room.

If you are going for color, make sure to use lighter shades and to incorporate lamps and lights here and there. (via biggerthanthethreeofus).

Use a large wall with no windows for storage like here: two big wardrobes and a file cabinet cover the whole wall.

The unfinished concrete walls not only keep the space light-colored but also bring texture to the bedroom. Lights are paired with a working fireplace.
Natural Touches
You can never go wrong with adding a bit of nature to your space, and with a windowless room it works even better. Make an indoor garden in this space but choose low-maintenance plants that can adapt to these conditions. Can’t find such plants? Stick to faux ones: they will add no less life while looking nice without any care.

The green colors of the room are highlighted with potted plants to make the space alive. (via stylebyemilyhenderson).

This is a large basement space divided into zones, and all of them are done in light shades. The plant life makes them fresh.