You pasted a theme, pressed enter, and your sidebar is still the same color. Or it changed, and an hour later it changed back. A Slack theme not working is almost never a bug in Slack. It's nearly always one of a handful of causes, and each has a fix that takes under a minute.
This guide is organized by symptom. Find the one that matches what you're seeing, try the fix, and move on. If you only need the normal steps for switching themes, those are in how to change your Slack theme. And if you need a known-good theme to test with, grab one from our list of the best Slack themes.
Thirty-Second Checklist
Most people can stop here.
- Are you looking at the workspace you pasted the theme into? Themes are set per workspace.
- Does your theme string have exactly eight hex colors, separated by commas, with no spaces or extra text?
- Is appearance set to Sync with system? That changes light and dark on your computer's schedule.
- Are you on your phone? Custom sidebar themes don't apply in the mobile apps.
- Have you fully quit and reopened Slack since the change?
Symptom 1: You Pasted a Theme and Nothing Happened
The string is malformed
This is the most common cause by a wide margin. Slack quietly ignores a custom theme it can't parse. A valid string is eight values, each a # followed by six hex characters, with commas between them:
#1A1D21,#2C2D30,#167891,#FFFFFF,#2C2D30,#D1D2D3,#8BD6D4,#FF5FA2
Check for the usual culprits: a missing value, a seven-character color, a space after a comma, a trailing comma, or quotation marks picked up when you copied it from a web page. Strings copied from chat apps and documents sometimes carry invisible formatting, so paste into a plain text editor first if a string refuses to work.
You pasted it in the wrong place
The string goes in Preferences, under the themes section, in the custom theme field. Pasting it into the message box does something different: Slack recognizes the format and offers a button to switch the sidebar theme. That button works, but you have to click it — the message alone changes nothing.
You're looking at a different workspace
If you belong to several workspaces, it's easy to change the theme in one and then stare at another. Check which workspace is selected before deciding the theme failed.
Symptom 2: Only Part of the Theme Changed
The sidebar changed but the message area didn't
This is expected. Slack has two separate settings. The appearance mode (light, dark, or sync with system) controls the main message area. The theme controls the sidebar. A dark sidebar string won't darken your messages; switch the mode to Dark as well. The differences are explained in Slack theme colors.
Some colors look different from the preview
Slack has changed how many colors a theme controls over the years, and it adjusts some shades automatically for contrast. Older eight-color strings still work, but a few elements may not match a screenshot taken in an earlier version. If a detail matters to you, tweak that one value and paste again.
Symptom 3: The Theme Keeps Changing or Resetting
Sync with system is switching modes for you
If Slack is light in the morning and dark in the evening, your operating system is changing its appearance on a schedule and Slack is following it. Open Preferences, go to the themes section, and choose Light or Dark explicitly instead of syncing with the system.
Another device or window applied a different theme
Your theme is stored with your account in that workspace. If you clicked a "switch sidebar theme" button on another computer, or in a browser tab you forgot about, it follows you. Paste the theme you want once more and close stray sessions.
You signed out, or the app was reinstalled
A fresh install or a new sign-in sometimes starts on the default sidebar. Keep your theme string somewhere easy to find. A message to yourself in Slack is the simplest place.
Symptom 4: The Theme Is Missing on Your Phone
This one isn't fixable, because it isn't broken. The iOS and Android apps let you choose light or dark appearance, and that's all. Custom sidebar strings aren't supported there, so there's nothing to sync from desktop. Your desktop theme is unaffected.
Symptom 5: The Theme Works in the App but Not the Browser
The desktop app and the browser version are separate clients, and each caches aggressively. If one shows the new theme and the other doesn't:
- In the browser, do a hard refresh with
Ctrl+Shift+Ron Windows and Linux orCmd+Shift+Ron Mac. - Check for a browser extension that restyles pages. Dark-mode extensions and custom-CSS tools can override Slack's colors. Disable them for Slack and reload.
- Confirm both are signed in to the same workspace with the same account.
Symptom 6: The Theme Options Aren't There at All
Slack is out of date
Theme settings have moved between versions. If your Preferences screen doesn't match current instructions, update the app. On desktop, open the Help menu and check for updates, or reinstall from Slack's download page.
You're looking in the old location
Themes have lived under different labels over time. Open Preferences with Ctrl+, or Cmd+, and look for the section about themes or appearance. The custom theme field may be collapsed; expand it.
The Reset That Fixes Almost Everything
When nothing above works, do the full sequence once, in order:
- Quit Slack completely. Closing the window leaves it running in the background. Use Quit from the menu or the system tray.
- Reopen and update if an update is offered.
- Clear the cache from the Help menu's troubleshooting options, and let Slack restart.
- Select a built-in theme first. If a built-in theme applies, Slack is working and your string is the problem.
- Paste a known-good string, such as the example near the top of this page.
- Then paste your own string. If it fails where the known-good one succeeded, count the values again.
That fourth step is the useful diagnostic. It separates "Slack won't change themes" from "this particular string is invalid," and it's the second one nearly every time.
Quick Reference
| What you see | Most likely cause | Fix |
|---|---|---|
| Nothing changes after pasting | Malformed string | Check for eight clean hex values |
| Sidebar changes, messages don't | Mode and theme are separate | Set Light or Dark as well |
| Theme flips during the day | Sync with system | Choose a mode explicitly |
| Theme reverts after sign-in | Fresh session | Paste again; save your string |
| No theme on phone | Not supported on mobile | Nothing to fix |
| App and browser disagree | Cache or extension | Hard refresh; disable extensions |
Once It Works
With the theme behaving, the fun part is choosing one. We keep collections for most tastes: Slack themes for developers (Dracula, Nord, Solarized and friends), Slack skins, and a seasonal Slack Christmas theme set. Share a string in a channel and teammates can apply it with one click, which is a surprisingly effective two-minute team ritual.
While you're tidying Slack, the rest of the workspace is worth ten minutes too. Our Slack tips and tricks and guide to sidebar sections cover the settings most people never find.
And if you'd like Slack to feel better as well as look better: Propsly (ours, to be clear) adds peer recognition with a single /props command. It's free for unlimited users, and it works with any theme.