diff --git a/res/img/03b381.png b/res/img/03b381.png
deleted file mode 100644
index cf28fc7e59..0000000000
Binary files a/res/img/03b381.png and /dev/null differ
diff --git a/res/img/368bd6.png b/res/img/368bd6.png
deleted file mode 100644
index a2700bd0ae..0000000000
Binary files a/res/img/368bd6.png and /dev/null differ
diff --git a/res/img/ac3ba8.png b/res/img/ac3ba8.png
deleted file mode 100644
index 031471d85a..0000000000
Binary files a/res/img/ac3ba8.png and /dev/null differ
diff --git a/res/themes/light-custom/css/_custom.scss b/res/themes/light-custom/css/_custom.scss
index e4a08277f9..64c4cac7e3 100644
--- a/res/themes/light-custom/css/_custom.scss
+++ b/res/themes/light-custom/css/_custom.scss
@@ -17,6 +17,7 @@ limitations under the License.
 //
 // --accent-color
 $accent-color: var(--accent-color);
+$accent-bg-color: var(--accent-color-15pct);
 $button-bg-color: var(--accent-color);
 $button-link-fg-color: var(--accent-color);
 $button-primary-bg-color: var(--accent-color);
@@ -124,3 +125,12 @@ $notice-primary-color: var(--warning-color);
 $pinned-unread-color: var(--warning-color);
 $warning-color: var(--warning-color);
 $button-danger-disabled-bg-color: var(--warning-color-50pct); // still needs alpha at 0.5
+
+$username-variant1-color: var(--username-colors_1, $username-variant1-color);
+$username-variant2-color: var(--username-colors_2, $username-variant2-color);
+$username-variant3-color: var(--username-colors_3, $username-variant3-color);
+$username-variant4-color: var(--username-colors_4, $username-variant4-color);
+$username-variant5-color: var(--username-colors_5, $username-variant5-color);
+$username-variant6-color: var(--username-colors_6, $username-variant6-color);
+$username-variant7-color: var(--username-colors_7, $username-variant7-color);
+$username-variant8-color: var(--username-colors_8, $username-variant8-color);
diff --git a/src/Avatar.js b/src/Avatar.js
index 217b196348..8393ce02b2 100644
--- a/src/Avatar.js
+++ b/src/Avatar.js
@@ -53,13 +53,56 @@ export function avatarUrlForUser(user, width, height, resizeMethod) {
     return url;
 }
 
+function isValidHexColor(color) {
+    return typeof color === "string" &&
+        (color.length === 7 || color.lengh === 9) &&
+        color.charAt(0) === "#" &&
+        !color.substr(1).split("").some(c => isNaN(parseInt(c, 16)));
+}
+
+function urlForColor(color) {
+    const size = 40;
+    const canvas = document.createElement("canvas");
+    canvas.width = size;
+    canvas.height = size;
+    const ctx = canvas.getContext("2d");
+    // bail out when using jsdom in unit tests
+    if (!ctx) {
+        return "";
+    }
+    ctx.fillStyle = color;
+    ctx.fillRect(0, 0, size, size);
+    return canvas.toDataURL();
+}
+
+// XXX: Ideally we'd clear this cache when the theme changes
+// but since this function is at global scope, it's a bit
+// hard to install a listener here, even if there were a clear event to listen to
+const colorToDataURLCache = new Map();
+
 export function defaultAvatarUrlForString(s) {
-    const images = ['03b381', '368bd6', 'ac3ba8'];
+    const defaultColors = ['#03b381', '#368bd6', '#ac3ba8'];
     let total = 0;
     for (let i = 0; i < s.length; ++i) {
         total += s.charCodeAt(i);
     }
-    return require('../res/img/' + images[total % images.length] + '.png');
+    const colorIndex = total % defaultColors.length;
+    // overwritten color value in custom themes
+    const cssVariable = `--avatar-background-colors_${colorIndex}`;
+    const cssValue = document.body.style.getPropertyValue(cssVariable);
+    const color = cssValue || defaultColors[colorIndex];
+    let dataUrl = colorToDataURLCache.get(color);
+    if (!dataUrl) {
+        // validate color as this can come from account_data
+        // with custom theming
+        if (isValidHexColor(color)) {
+            dataUrl = urlForColor(color);
+            colorToDataURLCache.set(color, dataUrl);
+        } else {
+            dataUrl = "";
+        }
+    }
+    return dataUrl;
 }
 
 /**
diff --git a/src/theme.js b/src/theme.js
index 442a89e25f..ab5974f207 100644
--- a/src/theme.js
+++ b/src/theme.js
@@ -141,16 +141,31 @@ export function enumerateThemes() {
     return Object.assign({}, customThemeNames, BUILTIN_THEMES);
 }
 
+
 function setCustomThemeVars(customTheme) {
     const {style} = document.body;
-    if (customTheme.colors) {
-        for (const [name, hexColor] of Object.entries(customTheme.colors)) {
-            style.setProperty(`--${name}`, hexColor);
-            // uses #rrggbbaa to define the color with alpha values at 0% and 50%
+
+    function setCSSVariable(name, hexColor, doPct = true) {
+        style.setProperty(`--${name}`, hexColor);
+        if (doPct) {
+            // uses #rrggbbaa to define the color with alpha values at 0%, 15% and 50%
             style.setProperty(`--${name}-0pct`, hexColor + "00");
+            style.setProperty(`--${name}-15pct`, hexColor + "26");
             style.setProperty(`--${name}-50pct`, hexColor + "7F");
         }
     }
+
+    if (customTheme.colors) {
+        for (const [name, value] of Object.entries(customTheme.colors)) {
+            if (Array.isArray(value)) {
+                for (let i = 0; i < value.length; i += 1) {
+                    setCSSVariable(`${name}_${i}`, value[i], false);
+                }
+            } else {
+                setCSSVariable(name, value);
+            }
+        }
+    }
 }
 
 function getCustomTheme(themeName) {