UI: Add grid-related theme options

This commit makes grid theming more consistent and capable by adding
some new theme colors related to grid rendering.
 - Add grid theme color for node editor. `UI_view2d_multi_grid_draw`
   is called with TH_GRID instead of a shaded `TH_BACK`.
   Also color-blend `TH_NODE_GROUP`.
 - Make the movie clip editor's clip preview grid respect grid theme
   color (`ED_region_grid_draw` uses color-blended `TH_GRID`).
 - Add versioning code to allow fixing existing themes (the resulting
   themes should visually look the same as before)

These changes did cause some inconsistencies in the movie clip editor,
even after adjusting the themes accordingly:
1. The alpha slider of the grid color affects the background and not
   the grid lines themselves.
2. The grids used by graph and dopesheet mode could already be themed
   in the past. Now that the clip preview's grid can also be themed,
   two different modes share the same theme color.

Differential Revision: https://developer.blender.org/D8699

1 files changed, 5 insertions, 2 deletions

diff --git a/source/blender/editors/interface/view2d.c b/source/blender/editors/interface/view2d.c
index 7651989c2df..9057ccaa762 100644
--- a/source/blender/editors/interface/view2d.c
+++ b/source/blender/editors/interface/view2d.c
@@ -1341,7 +1341,9 @@ void UI_view2d_multi_grid_draw(
   immBeginAtMost(GPU_PRIM_LINES, vertex_count);
 
   for (int level = 0; level < totlevels; level++) {
-    UI_GetThemeColorShade3ubv(colorid, offset, grid_line_color);
+    /* Blend the background color (colorid) with the grid color, to avoid either too low contrast
+     * or high contrast grid lines. This only has an effect if colorid != TH_GRID. */
+    UI_GetThemeColorBlendShade3ubv(colorid, TH_GRID, 0.25f, offset, grid_line_color);
 
     int i = (int)(v2d->cur.xmin / lstep);
     if (v2d->cur.xmin > 0.0f) {
@@ -1382,7 +1384,8 @@ void UI_view2d_multi_grid_draw(
   }
 
   /* X and Y axis */
-  UI_GetThemeColorShade3ubv(colorid, -18 + ((totlevels - 1) * -6), grid_line_color);
+  UI_GetThemeColorBlendShade3ubv(
+      colorid, TH_GRID, 0.5f, -18 + ((totlevels - 1) * -6), grid_line_color);
 
   immAttrSkip(color);
   immVertex2f(pos, 0.0f, v2d->cur.ymin);