UI: Use scrubbing theme setting for markers background.

The hard-coded transparency of just 16 made it hard to see the markers
when the background was busy with keyframes (or strips in VSE).

The rename of the setting is in the following commit.

Reviewers: billreynish

source/blender/editors/animation/anim_markers.c

@@ -481,7 +481,9 @@ static void draw_markers_background(rctf *rect)
   uint pos = GPU_vertformat_attr_add(immVertexFormat(), "pos", GPU_COMP_F32, 2, GPU_FETCH_FLOAT);
   immBindBuiltinProgram(GPU_SHADER_2D_UNIFORM_COLOR);
 
-  const unsigned char shade[4] = {0, 0, 0, 16};
+  const unsigned char shade[4];
+  UI_GetThemeColor4ubv(TH_SCRUBBING_BACKGROUND, shade);
+
   immUniformColor4ubv(shade);
 
   GPU_blend(true);