There's a temptation to just pile on adjectives, but I've found the specific sensory detail does way more heavy lifting. Instead of saying a room was 'spooky,' describe the way the dust motes hang frozen in a single slant of pale light, or how the floorboards give a little sigh underfoot as if remembering the weight of someone else. That immediate, tangible sensation grounds the reader in the mood you're building before you even name the emotion. I tend to highlight one sense per scene at the start, maybe the scent of wet earth after rain to underscore melancholy, or the grating, metallic screech of a train for anxiety. It's like handing the reader the raw materials and letting them assemble the feeling themselves, which sticks far longer than a label.
Dialogue tags and action beats are a stealth tool, too. A character can say something 'coldly,' or you can write 'She set her cup down without a sound, the porcelain meeting the wood with a precision that felt like a verdict.' The latter builds the tense atmosphere through the action. The narrative voice itself should have a rhythm that matches the tone; short, staccato sentences for urgency, longer, flowing ones for contemplation or dread. It's the difference between being told a story is dark and feeling the shadows gather around you as you read.