First version with naive color extrapolation.

src/components/settings/settings.js

@@ -10,7 +10,10 @@ const settings = {
       muteWordsString: this.$store.state.config.muteWords.join('\n'),
       autoLoadLocal: this.$store.state.config.autoLoad,
       streamingLocal: this.$store.state.config.streaming,
-      hoverPreviewLocal: this.$store.state.config.hoverPreview
+      hoverPreviewLocal: this.$store.state.config.hoverPreview,
+      bgColorLocal: '',
+      fgColorLocal: '',
+      linkColorLocal: ''
     }
   },
   components: {
@@ -21,6 +24,32 @@ const settings = {
       return this.$store.state.users.currentUser
     }
   },
+  methods: {
+    setCustomTheme () {
+      if (!this.bgColorLocal && !this.fgColorLocal && !this.linkColorLocal) {
+        // reset to picked themes
+      }
+      const rgb = (hex) => {
+        const result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
+        return result ? {
+            r: parseInt(result[1], 16),
+            g: parseInt(result[2], 16),
+            b: parseInt(result[3], 16)
+        } : null
+      }
+      const bgRgb = rgb(this.bgColorLocal)
+      const fgRgb = rgb(this.fgColorLocal)
+      const linkRgb = rgb(this.linkColorLocal)
+      if (bgRgb && fgRgb && linkRgb) {
+        console.log('all colors ok')
+        this.$store.dispatch('setOption', { name: 'customTheme', value: {
+          fg: fgRgb,
+          bg: bgRgb,
+          link: linkRgb
+        }})
+      }
+    }
+  },
   watch: {
     hideAttachmentsLocal (value) {
       this.$store.dispatch('setOption', { name: 'hideAttachments', value })

src/components/settings/settings.vue

@@ -8,6 +8,14 @@
         <h2>{{$t('settings.theme')}}</h2>
         <style-switcher></style-switcher>
       </div>
+      <div class="setting-item">
+        <h3>Custom theme</h3>
+        <p>Enter hex color codes (#aabbcc) into the text fields.</p>
+        <input type="text" placeholder="Background" v-model="bgColorLocal">
+        <input type="text" placeholder="Foreground" v-model="fgColorLocal">
+        <input type="text" placeholder="Highlight" v-model="linkColorLocal">
+        <button @click="setCustomTheme">Submit</button>
+      </div>
       <div class="setting-item">
         <h2>{{$t('settings.filtering')}}</h2>
         <p>{{$t('settings.filtering_explanation')}}</p>