qgisqgis-3
I'm trying to remove all strokes of boundary of categories by selecting
"Symobol selector"->"Stroke style"->"No pen"
However, if there are a large amount of symbols, it is impossible to change all.
Are there any measures to change all symbols at one time?
I am assuming that these small rings are "donut holes", or gaps within the polygon (and am mostly writing this for better search result hits, as the title is a bit ambiguous to me).
My solution to this using GRASS GIS is as follows:
# (optional before starting: clean features
v.clean in=original_import out=original_map tool=bpol,rmdup
v.build map=original_map
# 1. Add centroids to the holes
v.centroids in=original_map out=map_with_centroids
# 2. Drop the current table and and one that includes the
# areas that now have centroids and a value column that
# you will use to dissolve the areas
v.db.droptable map=map_with_centroids -f
v.db.addtable map=map_with_centroids columns='value INT'
# 3. Update this "value" column with the same value everywhere
v.db.update map=map_with_centroids col=value value=1
# 4. Now combine the areas by dissolving them together
v.dissolve in=map_with_centroids out=map_noholes column=value
If you rename "original import" to your input map name and "map_noholes" to your desired output map name, you should be able to just copy/paste this block.
Select your symbols by either clicking them individually whilst holding Ctrl or click the uppermost symbol, hold Shift and click the last symbol you want selected. Then go to Symbol > Change:
Then select your fill and set the border style:
When you click OK, the selected symbols should be updated with the new border properties:
EDIT:
To keep your pattern fills, a possible workaround is after you have categorised your layer, change it to Rule-based. This will create expressions for each of your symbol which should match the categorised version. Then create a new rule which has a transparent fill and set your border with the required settings and with no filter/rule:
You should now have features which contain your pattern fill but with the border you have set:
Best Answer
In "Symbology", if you select all symbols, then click in the "Symbol" area (in your screenshot it was in green), then
Simple Fill, and select aStroke Width(or any other style), it will be applied to all symbols.