Made Timeline & Tool buttons change depending on theme type

Changed "LayerButtons" group to "UIButtons", as it includes most UI buttons.

Assets/Graphics/Dark Themes/Timeline/Next_Frame_hover.png.import

@@ -2,15 +2,15 @@
 
 importer="texture"
 type="StreamTexture"
-path="res://.import/Next_Frame_hover.png-29e3bdc3bfb09dbf24f0d4ef57cdadd0.stex"
+path="res://.import/Next_Frame_Hover.png-ba92cf8a5dbcda245610c78c96d6bf93.stex"
 metadata={
 "vram_texture": false
 }
 
 [deps]
 
-source_file="res://Assets/Graphics/Dark Themes/Timeline/Next_Frame_hover.png"
-dest_files=[ "res://.import/Next_Frame_hover.png-29e3bdc3bfb09dbf24f0d4ef57cdadd0.stex" ]
+source_file="res://Assets/Graphics/Dark Themes/Timeline/Next_Frame_Hover.png"
+dest_files=[ "res://.import/Next_Frame_Hover.png-ba92cf8a5dbcda245610c78c96d6bf93.stex" ]
 
 [params]
 

Assets/Graphics/Dark Themes/Timeline/onion_skinning.png.import

@@ -1,34 +0,0 @@
-[remap]
-
-importer="texture"
-type="StreamTexture"
-path="res://.import/onion_skinning.png-d0842f2eb16b7f7b21f3177f2d045336.stex"
-metadata={
-"vram_texture": false
-}
-
-[deps]
-
-source_file="res://Assets/Graphics/Dark Themes/Timeline/onion_skinning.png"
-dest_files=[ "res://.import/onion_skinning.png-d0842f2eb16b7f7b21f3177f2d045336.stex" ]
-
-[params]
-
-compress/mode=0
-compress/lossy_quality=0.7
-compress/hdr_mode=0
-compress/bptc_ldr=0
-compress/normal_map=0
-flags/repeat=0
-flags/filter=false
-flags/mipmaps=false
-flags/anisotropic=false
-flags/srgb=2
-process/fix_alpha_border=true
-process/premult_alpha=false
-process/HDR_as_SRGB=false
-process/invert_color=false
-stream=false
-size_limit=0
-detect_3d=true
-svg/scale=1.0