style(themes): increase text contrast and darken secondary color

- update text color palette in dark theme for higher contrast (pure white base)
- update text color palette in light theme for higher contrast (pure black base)
- darken `--color-secondary` in dark theme from #30363d to #1b1b1b
- add `DEFAULT_THEME = octo-auto` example to README

README.md

@@ -25,6 +25,7 @@ Thème Gitea épuré, palette neutre, disponible en variante claire et sombre.
    ```ini
    [ui]
    THEMES = gitea-auto, gitea-light, gitea-dark, octo-auto, octo-light, octo-dark
+   DEFAULT_THEME = octo-auto
    ```
 
    Les noms à ajouter sont **sans** le préfixe `theme-` ni le suffixe `.css`.

themes/theme-octo-dark.css

@@ -32,7 +32,7 @@ gitea-theme-meta-info {
   --color-primary-alpha-90: #1f6febe1;
   --color-primary-hover: var(--color-primary-light-1);
   --color-primary-active: var(--color-primary-light-2);
-  --color-secondary: #30363d;
+  --color-secondary: #1b1b1b;
   --color-secondary-dark-1: #3d444d;
   --color-secondary-dark-2: #484f58;
   --color-secondary-dark-3: #586069;
@@ -192,12 +192,12 @@ gitea-theme-meta-info {
   --color-box-header: #000000;
   --color-box-body: #020202;
   --color-box-body-highlight: #09090a;
-  --color-text-dark: #e6edf3;
-  --color-text: #c9d1d9;
-  --color-text-light: #adbac7;
-  --color-text-light-1: #8b949e;
-  --color-text-light-2: #6e7681;
-  --color-text-light-3: #484f58;
+  --color-text-dark: #ffffff;
+  --color-text: #ffffff;
+  --color-text-light: #dddddd;
+  --color-text-light-1: #c4c4c4;
+  --color-text-light-2: #a7a7a7;
+  --color-text-light-3: #868686;
   --color-footer: var(--color-nav-bg);
   --color-timeline: #21262d;
   --color-input-text: var(--color-text-dark);

themes/theme-octo-light.css

@@ -192,12 +192,12 @@ gitea-theme-meta-info {
   --color-box-header: #ffffff;
   --color-box-body: #ffffff;
   --color-box-body-highlight: #fafbfc;
-  --color-text-dark: #24292e;
-  --color-text: #444d56;
-  --color-text-light: #586069;
-  --color-text-light-1: #6a737d;
-  --color-text-light-2: #6a737d;
-  --color-text-light-3: #959da5;
+  --color-text-dark: #000000;
+  --color-text: #000000;
+  --color-text-light: #353535;
+  --color-text-light-1: #616161;
+  --color-text-light-2: #808080;
+  --color-text-light-3: #9c9c9c;
   --color-footer: var(--color-nav-bg);
   --color-timeline: #e1e4e8;
   --color-input-text: var(--color-text-dark);