Themes: Add a Materia Gruvbox theme

.local/share/themes/Materia-Gruvbox/metacity-1/assets/button.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <circle cx="12" cy="12" r="12"/>
+</svg>

.local/share/themes/Materia-Gruvbox/metacity-1/assets/close.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <path d="M17 8.41L15.59 7 12 10.59 8.41 7 7 8.41 10.59 12 7 15.59 8.41 17 12 13.41 15.59 17 17 15.59 13.41 12z"/>
+</svg>

.local/share/themes/Materia-Gruvbox/metacity-1/assets/maximize.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <path d="m8 8v8h8v-8h-8zm2 2h4v4h-4v-4z"/>
+</svg>

.local/share/themes/Materia-Gruvbox/metacity-1/assets/menu.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <circle cx="12" cy="12" r="2"/>
+</svg>

.local/share/themes/Materia-Gruvbox/metacity-1/assets/minimize.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <path d="m8 14h8v2h-8z"/>
+</svg>

.local/share/themes/Materia-Gruvbox/metacity-1/assets/shade.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <path d="m6 14 1.41 1.41 4.59-4.59 4.59 4.59 1.41-1.41-6-6z"/>
+</svg>

.local/share/themes/Materia-Gruvbox/metacity-1/assets/unmaximize.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <path d="m9 9v6h6v-6h-6zm2 2h2v2h-2v-2z"/>
+</svg>

.local/share/themes/Materia-Gruvbox/metacity-1/assets/unshade.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="#808080">
+  <path d="m18 10-1.41-1.41-4.59 4.59-4.59-4.59-1.41 1.41l6 6z"/>
+</svg>