This calls glColor indirectly, so must be changed. You can use immUniformThemeColorShade after binding GPU_SHADER_2D_UNIFORM_COLOR.

Since you're using uniform color, do this once after immBind...

immUniformColor4f(0.0f, 0.0f, 0.0f, 0.4f);

Then call immRectf below.

We were thinking the same thing!

Now this color attribute is not needed.

immUniformThemeColor(TH_CFRAME);

here instead of

UI_ThemeColor(TH_CFRAME);

above.